django/tests/schema/tests.py

import datetime
import itertools
import unittest
from copy import copy
from decimal import Decimal
from unittest import mock

from django.core.exceptions import FieldError
from django.core.management.color import no_style
from django.core.serializers.json import DjangoJSONEncoder
from django.db import (
    DatabaseError,
    DataError,
    IntegrityError,
    OperationalError,
    connection,
)
from django.db.backends.utils import truncate_name
from django.db.models import (
    CASCADE,
    PROTECT,
    AutoField,
    BigAutoField,
    BigIntegerField,
    BinaryField,
    BooleanField,
    CharField,
    CheckConstraint,
    DateField,
    DateTimeField,
    DecimalField,
    DurationField,
    F,
    FloatField,
    ForeignKey,
    ForeignObject,
    GeneratedField,
    Index,
    IntegerField,
    JSONField,
    ManyToManyField,
    Model,
    OneToOneField,
    OrderBy,
    PositiveIntegerField,
    Q,
    SlugField,
    SmallAutoField,
    SmallIntegerField,
    TextField,
    TimeField,
    UniqueConstraint,
    UUIDField,
    Value,
)
from django.db.models.fields.json import KT, KeyTextTransform
from django.db.models.functions import Abs, Cast, Collate, Lower, Random, Round, Upper
from django.db.models.indexes import IndexExpression
from django.db.transaction import TransactionManagementError, atomic
from django.test import (
    TransactionTestCase,
    ignore_warnings,
    skipIfDBFeature,
    skipUnlessDBFeature,
)
from django.test.utils import CaptureQueriesContext, isolate_apps, register_lookup
from django.utils.deprecation import RemovedInDjango51Warning

from .fields import CustomManyToManyField, InheritedManyToManyField, MediumBlobField
from .models import (
    Author,
    AuthorCharFieldWithIndex,
    AuthorTextFieldWithIndex,
    AuthorWithDefaultHeight,
    AuthorWithEvenLongerName,
    AuthorWithIndexedName,
    AuthorWithUniqueName,
    AuthorWithUniqueNameAndBirthday,
    Book,
    BookForeignObj,
    BookWeak,
    BookWithLongName,
    BookWithO2O,
    BookWithoutAuthor,
    BookWithSlug,
    IntegerPK,
    Node,
    Note,
    NoteRename,
    Tag,
    TagM2MTest,
    TagUniqueRename,
    Thing,
    UniqueTest,
    new_apps,
)


class SchemaTests(TransactionTestCase):
    """
    Tests for the schema-alteration code.

    Be aware that these tests are more liable than most to false results,
    as sometimes the code to check if a test has worked is almost as complex
    as the code it is testing.
    """

    available_apps = []

    models = [
        Author,
        AuthorCharFieldWithIndex,
        AuthorTextFieldWithIndex,
        AuthorWithDefaultHeight,
        AuthorWithEvenLongerName,
        Book,
        BookWeak,
        BookWithLongName,
        BookWithO2O,
        BookWithSlug,
        IntegerPK,
        Node,
        Note,
        Tag,
        TagM2MTest,
        TagUniqueRename,
        Thing,
        UniqueTest,
    ]

    # Utility functions

    def setUp(self):
        # local_models should contain test dependent model classes that will be
        # automatically removed from the app cache on test tear down.
        self.local_models = []
        # isolated_local_models contains models that are in test methods
        # decorated with @isolate_apps.
        self.isolated_local_models = []

    def tearDown(self):
        # Delete any tables made for our models
        self.delete_tables()
        new_apps.clear_cache()
        for model in new_apps.get_models():
            model._meta._expire_cache()
        if "schema" in new_apps.all_models:
            for model in self.local_models:
                for many_to_many in model._meta.many_to_many:
                    through = many_to_many.remote_field.through
                    if through and through._meta.auto_created:
                        del new_apps.all_models["schema"][through._meta.model_name]
                del new_apps.all_models["schema"][model._meta.model_name]
        if self.isolated_local_models:
            with connection.schema_editor() as editor:
                for model in self.isolated_local_models:
                    editor.delete_model(model)

    def delete_tables(self):
        "Deletes all model tables for our models for a clean test environment"
        converter = connection.introspection.identifier_converter
        with connection.schema_editor() as editor:
            connection.disable_constraint_checking()
            table_names = connection.introspection.table_names()
            if connection.features.ignores_table_name_case:
                table_names = [table_name.lower() for table_name in table_names]
            for model in itertools.chain(SchemaTests.models, self.local_models):
                tbl = converter(model._meta.db_table)
                if connection.features.ignores_table_name_case:
                    tbl = tbl.lower()
                if tbl in table_names:
                    editor.delete_model(model)
                    table_names.remove(tbl)
            connection.enable_constraint_checking()

    def column_classes(self, model):
        with connection.cursor() as cursor:
            columns = {
                d[0]: (connection.introspection.get_field_type(d[1], d), d)
                for d in connection.introspection.get_table_description(
                    cursor,
                    model._meta.db_table,
                )
            }
        # SQLite has a different format for field_type
        for name, (type, desc) in columns.items():
            if isinstance(type, tuple):
                columns[name] = (type[0], desc)
        return columns

    def get_primary_key(self, table):
        with connection.cursor() as cursor:
            return connection.introspection.get_primary_key_column(cursor, table)

    def get_indexes(self, table):
        """
        Get the indexes on the table using a new cursor.
        """
        with connection.cursor() as cursor:
            return [
                c["columns"][0]
                for c in connection.introspection.get_constraints(
                    cursor, table
                ).values()
                if c["index"] and len(c["columns"]) == 1
            ]

    def get_uniques(self, table):
        with connection.cursor() as cursor:
            return [
                c["columns"][0]
                for c in connection.introspection.get_constraints(
                    cursor, table
                ).values()
                if c["unique"] and len(c["columns"]) == 1
            ]

    def get_constraints(self, table):
        """
        Get the constraints on a table using a new cursor.
        """
        with connection.cursor() as cursor:
            return connection.introspection.get_constraints(cursor, table)

    def get_constraints_for_column(self, model, column_name):
        constraints = self.get_constraints(model._meta.db_table)
        constraints_for_column = []
        for name, details in constraints.items():
            if details["columns"] == [column_name]:
                constraints_for_column.append(name)
        return sorted(constraints_for_column)

    def get_constraint_opclasses(self, constraint_name):
        with connection.cursor() as cursor:
            sql = """
                SELECT opcname
                FROM pg_opclass AS oc
                JOIN pg_index as i on oc.oid = ANY(i.indclass)
                JOIN pg_class as c on c.oid = i.indexrelid
                WHERE c.relname = %s
            """
            cursor.execute(sql, [constraint_name])
            return [row[0] for row in cursor.fetchall()]

    def check_added_field_default(
        self,
        schema_editor,
        model,
        field,
        field_name,
        expected_default,
        cast_function=None,
    ):
        with connection.cursor() as cursor:
            schema_editor.add_field(model, field)
            cursor.execute(
                "SELECT {} FROM {};".format(field_name, model._meta.db_table)
            )
            database_default = cursor.fetchall()[0][0]
            if cast_function and type(database_default) is not type(expected_default):
                database_default = cast_function(database_default)
            self.assertEqual(database_default, expected_default)

    def get_constraints_count(self, table, column, fk_to):
        """
        Return a dict with keys 'fks', 'uniques, and 'indexes' indicating the
        number of foreign keys, unique constraints, and indexes on
        `table`.`column`. The `fk_to` argument is a 2-tuple specifying the
        expected foreign key relationship's (table, column).
        """
        with connection.cursor() as cursor:
            constraints = connection.introspection.get_constraints(cursor, table)
        counts = {"fks": 0, "uniques": 0, "indexes": 0}
        for c in constraints.values():
            if c["columns"] == [column]:
                if c["foreign_key"] == fk_to:
                    counts["fks"] += 1
                if c["unique"]:
                    counts["uniques"] += 1
                elif c["index"]:
                    counts["indexes"] += 1
        return counts

    def get_column_collation(self, table, column):
        with connection.cursor() as cursor:
            return next(
                f.collation
                for f in connection.introspection.get_table_description(cursor, table)
                if f.name == column
            )

    def get_column_comment(self, table, column):
        with connection.cursor() as cursor:
            return next(
                f.comment
                for f in connection.introspection.get_table_description(cursor, table)
                if f.name == column
            )

    def get_table_comment(self, table):
        with connection.cursor() as cursor:
            return next(
                t.comment
                for t in connection.introspection.get_table_list(cursor)
                if t.name == table
            )

    def assert_column_comment_not_exists(self, table, column):
        with connection.cursor() as cursor:
            columns = connection.introspection.get_table_description(cursor, table)
        self.assertFalse(any([c.name == column and c.comment for c in columns]))

    def assertIndexOrder(self, table, index, order):
        constraints = self.get_constraints(table)
        self.assertIn(index, constraints)
        index_orders = constraints[index]["orders"]
        self.assertTrue(
            all(val == expected for val, expected in zip(index_orders, order))
        )

    def assertForeignKeyExists(self, model, column, expected_fk_table, field="id"):
        """
        Fail if the FK constraint on `model.Meta.db_table`.`column` to
        `expected_fk_table`.id doesn't exist.
        """
        if not connection.features.can_introspect_foreign_keys:
            return
        constraints = self.get_constraints(model._meta.db_table)
        constraint_fk = None
        for details in constraints.values():
            if details["columns"] == [column] and details["foreign_key"]:
                constraint_fk = details["foreign_key"]
                break
        self.assertEqual(constraint_fk, (expected_fk_table, field))

    def assertForeignKeyNotExists(self, model, column, expected_fk_table):
        if not connection.features.can_introspect_foreign_keys:
            return
        with self.assertRaises(AssertionError):
            self.assertForeignKeyExists(model, column, expected_fk_table)

    # Tests
    def test_creation_deletion(self):
        """
        Tries creating a model's table, and then deleting it.
        """
        with connection.schema_editor() as editor:
            # Create the table
            editor.create_model(Author)
            # The table is there
            list(Author.objects.all())
            # Clean up that table
            editor.delete_model(Author)
            # No deferred SQL should be left over.
            self.assertEqual(editor.deferred_sql, [])
        # The table is gone
        with self.assertRaises(DatabaseError):
            list(Author.objects.all())

    @skipUnlessDBFeature("supports_foreign_keys")
    def test_fk(self):
        "Creating tables out of FK order, then repointing, works"
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Book)
            editor.create_model(Author)
            editor.create_model(Tag)
        # Initial tables are there
        list(Author.objects.all())
        list(Book.objects.all())
        # Make sure the FK constraint is present
        with self.assertRaises(IntegrityError):
            Book.objects.create(
                author_id=1,
                title="Much Ado About Foreign Keys",
                pub_date=datetime.datetime.now(),
            )
        # Repoint the FK constraint
        old_field = Book._meta.get_field("author")
        new_field = ForeignKey(Tag, CASCADE)
        new_field.set_attributes_from_name("author")
        with connection.schema_editor() as editor:
            editor.alter_field(Book, old_field, new_field, strict=True)
        self.assertForeignKeyExists(Book, "author_id", "schema_tag")

    @skipUnlessDBFeature("can_create_inline_fk")
    def test_inline_fk(self):
        # Create some tables.
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
            editor.create_model(Note)
        self.assertForeignKeyNotExists(Note, "book_id", "schema_book")
        # Add a foreign key from one to the other.
        with connection.schema_editor() as editor:
            new_field = ForeignKey(Book, CASCADE)
            new_field.set_attributes_from_name("book")
            editor.add_field(Note, new_field)
        self.assertForeignKeyExists(Note, "book_id", "schema_book")
        # Creating a FK field with a constraint uses a single statement without
        # a deferred ALTER TABLE.
        self.assertFalse(
            [
                sql
                for sql in (str(statement) for statement in editor.deferred_sql)
                if sql.startswith("ALTER TABLE") and "ADD CONSTRAINT" in sql
            ]
        )

    @skipUnlessDBFeature("can_create_inline_fk")
    def test_add_inline_fk_update_data(self):
        with connection.schema_editor() as editor:
            editor.create_model(Node)
        # Add an inline foreign key and update data in the same transaction.
        new_field = ForeignKey(Node, CASCADE, related_name="new_fk", null=True)
        new_field.set_attributes_from_name("new_parent_fk")
        parent = Node.objects.create()
        with connection.schema_editor() as editor:
            editor.add_field(Node, new_field)
            editor.execute("UPDATE schema_node SET new_parent_fk_id = %s;", [parent.pk])
        assertIndex = (
            self.assertIn
            if connection.features.indexes_foreign_keys
            else self.assertNotIn
        )
        assertIndex("new_parent_fk_id", self.get_indexes(Node._meta.db_table))

    @skipUnlessDBFeature(
        "can_create_inline_fk",
        "allows_multiple_constraints_on_same_fields",
    )
    @isolate_apps("schema")
    def test_add_inline_fk_index_update_data(self):
        class Node(Model):
            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(Node)
        # Add an inline foreign key, update data, and an index in the same
        # transaction.
        new_field = ForeignKey(Node, CASCADE, related_name="new_fk", null=True)
        new_field.set_attributes_from_name("new_parent_fk")
        parent = Node.objects.create()
        with connection.schema_editor() as editor:
            editor.add_field(Node, new_field)
            Node._meta.add_field(new_field)
            editor.execute("UPDATE schema_node SET new_parent_fk_id = %s;", [parent.pk])
            editor.add_index(
                Node, Index(fields=["new_parent_fk"], name="new_parent_inline_fk_idx")
            )
        self.assertIn("new_parent_fk_id", self.get_indexes(Node._meta.db_table))

    @skipUnlessDBFeature("supports_foreign_keys")
    def test_char_field_with_db_index_to_fk(self):
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(AuthorCharFieldWithIndex)
        # Change CharField to FK
        old_field = AuthorCharFieldWithIndex._meta.get_field("char_field")
        new_field = ForeignKey(Author, CASCADE, blank=True)
        new_field.set_attributes_from_name("char_field")
        with connection.schema_editor() as editor:
            editor.alter_field(
                AuthorCharFieldWithIndex, old_field, new_field, strict=True
            )
        self.assertForeignKeyExists(
            AuthorCharFieldWithIndex, "char_field_id", "schema_author"
        )

    @skipUnlessDBFeature("supports_foreign_keys")
    @skipUnlessDBFeature("supports_index_on_text_field")
    def test_text_field_with_db_index_to_fk(self):
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(AuthorTextFieldWithIndex)
        # Change TextField to FK
        old_field = AuthorTextFieldWithIndex._meta.get_field("text_field")
        new_field = ForeignKey(Author, CASCADE, blank=True)
        new_field.set_attributes_from_name("text_field")
        with connection.schema_editor() as editor:
            editor.alter_field(
                AuthorTextFieldWithIndex, old_field, new_field, strict=True
            )
        self.assertForeignKeyExists(
            AuthorTextFieldWithIndex, "text_field_id", "schema_author"
        )

    @isolate_apps("schema")
    def test_char_field_pk_to_auto_field(self):
        class Foo(Model):
            id = CharField(max_length=255, primary_key=True)

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(Foo)
        self.isolated_local_models = [Foo]
        old_field = Foo._meta.get_field("id")
        new_field = AutoField(primary_key=True)
        new_field.set_attributes_from_name("id")
        new_field.model = Foo
        with connection.schema_editor() as editor:
            editor.alter_field(Foo, old_field, new_field, strict=True)

    @skipUnlessDBFeature("supports_foreign_keys")
    def test_fk_to_proxy(self):
        "Creating a FK to a proxy model creates database constraints."

        class AuthorProxy(Author):
            class Meta:
                app_label = "schema"
                apps = new_apps
                proxy = True

        class AuthorRef(Model):
            author = ForeignKey(AuthorProxy, on_delete=CASCADE)

            class Meta:
                app_label = "schema"
                apps = new_apps

        self.local_models = [AuthorProxy, AuthorRef]

        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(AuthorRef)
        self.assertForeignKeyExists(AuthorRef, "author_id", "schema_author")

    @skipUnlessDBFeature("supports_foreign_keys", "can_introspect_foreign_keys")
    def test_fk_db_constraint(self):
        "The db_constraint parameter is respected"
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Tag)
            editor.create_model(Author)
            editor.create_model(BookWeak)
        # Initial tables are there
        list(Author.objects.all())
        list(Tag.objects.all())
        list(BookWeak.objects.all())
        self.assertForeignKeyNotExists(BookWeak, "author_id", "schema_author")
        # Make a db_constraint=False FK
        new_field = ForeignKey(Tag, CASCADE, db_constraint=False)
        new_field.set_attributes_from_name("tag")
        with connection.schema_editor() as editor:
            editor.add_field(Author, new_field)
        self.assertForeignKeyNotExists(Author, "tag_id", "schema_tag")
        # Alter to one with a constraint
        new_field2 = ForeignKey(Tag, CASCADE)
        new_field2.set_attributes_from_name("tag")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, new_field, new_field2, strict=True)
        self.assertForeignKeyExists(Author, "tag_id", "schema_tag")
        # Alter to one without a constraint again
        new_field2 = ForeignKey(Tag, CASCADE)
        new_field2.set_attributes_from_name("tag")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, new_field2, new_field, strict=True)
        self.assertForeignKeyNotExists(Author, "tag_id", "schema_tag")

    @isolate_apps("schema")
    def test_no_db_constraint_added_during_primary_key_change(self):
        """
        When a primary key that's pointed to by a ForeignKey with
        db_constraint=False is altered, a foreign key constraint isn't added.
        """

        class Author(Model):
            class Meta:
                app_label = "schema"

        class BookWeak(Model):
            author = ForeignKey(Author, CASCADE, db_constraint=False)

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(BookWeak)
        self.assertForeignKeyNotExists(BookWeak, "author_id", "schema_author")
        old_field = Author._meta.get_field("id")
        new_field = BigAutoField(primary_key=True)
        new_field.model = Author
        new_field.set_attributes_from_name("id")
        # @isolate_apps() and inner models are needed to have the model
        # relations populated, otherwise this doesn't act as a regression test.
        self.assertEqual(len(new_field.model._meta.related_objects), 1)
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        self.assertForeignKeyNotExists(BookWeak, "author_id", "schema_author")

    def _test_m2m_db_constraint(self, M2MFieldClass):
        class LocalAuthorWithM2M(Model):
            name = CharField(max_length=255)

            class Meta:
                app_label = "schema"
                apps = new_apps

        self.local_models = [LocalAuthorWithM2M]

        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Tag)
            editor.create_model(LocalAuthorWithM2M)
        # Initial tables are there
        list(LocalAuthorWithM2M.objects.all())
        list(Tag.objects.all())
        # Make a db_constraint=False FK
        new_field = M2MFieldClass(Tag, related_name="authors", db_constraint=False)
        new_field.contribute_to_class(LocalAuthorWithM2M, "tags")
        # Add the field
        with connection.schema_editor() as editor:
            editor.add_field(LocalAuthorWithM2M, new_field)
        self.assertForeignKeyNotExists(
            new_field.remote_field.through, "tag_id", "schema_tag"
        )

    @skipUnlessDBFeature("supports_foreign_keys")
    def test_m2m_db_constraint(self):
        self._test_m2m_db_constraint(ManyToManyField)

    @skipUnlessDBFeature("supports_foreign_keys")
    def test_m2m_db_constraint_custom(self):
        self._test_m2m_db_constraint(CustomManyToManyField)

    @skipUnlessDBFeature("supports_foreign_keys")
    def test_m2m_db_constraint_inherited(self):
        self._test_m2m_db_constraint(InheritedManyToManyField)

    def test_add_field(self):
        """
        Tests adding fields to models
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Ensure there's no age field
        columns = self.column_classes(Author)
        self.assertNotIn("age", columns)
        # Add the new field
        new_field = IntegerField(null=True)
        new_field.set_attributes_from_name("age")
        with (
            CaptureQueriesContext(connection) as ctx,
            connection.schema_editor() as editor,
        ):
            editor.add_field(Author, new_field)
        drop_default_sql = editor.sql_alter_column_no_default % {
            "column": editor.quote_name(new_field.name),
        }
        self.assertFalse(
            any(drop_default_sql in query["sql"] for query in ctx.captured_queries)
        )
        # Table is not rebuilt.
        self.assertIs(
            any("CREATE TABLE" in query["sql"] for query in ctx.captured_queries), False
        )
        self.assertIs(
            any("DROP TABLE" in query["sql"] for query in ctx.captured_queries), False
        )
        columns = self.column_classes(Author)
        self.assertEqual(
            columns["age"][0],
            connection.features.introspected_field_types["IntegerField"],
        )
        self.assertTrue(columns["age"][1][6])

    def test_add_field_remove_field(self):
        """
        Adding a field and removing it removes all deferred sql referring to it.
        """
        with connection.schema_editor() as editor:
            # Create a table with a unique constraint on the slug field.
            editor.create_model(Tag)
            # Remove the slug column.
            editor.remove_field(Tag, Tag._meta.get_field("slug"))
        self.assertEqual(editor.deferred_sql, [])

    def test_add_field_temp_default(self):
        """
        Tests adding fields to models with a temporary default
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Ensure there's no age field
        columns = self.column_classes(Author)
        self.assertNotIn("age", columns)
        # Add some rows of data
        Author.objects.create(name="Andrew", height=30)
        Author.objects.create(name="Andrea")
        # Add a not-null field
        new_field = CharField(max_length=30, default="Godwin")
        new_field.set_attributes_from_name("surname")
        with connection.schema_editor() as editor:
            editor.add_field(Author, new_field)
        columns = self.column_classes(Author)
        self.assertEqual(
            columns["surname"][0],
            connection.features.introspected_field_types["CharField"],
        )
        self.assertEqual(
            columns["surname"][1][6],
            connection.features.interprets_empty_strings_as_nulls,
        )

    def test_add_field_temp_default_boolean(self):
        """
        Tests adding fields to models with a temporary default where
        the default is False. (#21783)
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Ensure there's no age field
        columns = self.column_classes(Author)
        self.assertNotIn("age", columns)
        # Add some rows of data
        Author.objects.create(name="Andrew", height=30)
        Author.objects.create(name="Andrea")
        # Add a not-null field
        new_field = BooleanField(default=False)
        new_field.set_attributes_from_name("awesome")
        with connection.schema_editor() as editor:
            editor.add_field(Author, new_field)
        columns = self.column_classes(Author)
        # BooleanField are stored as TINYINT(1) on MySQL.
        field_type = columns["awesome"][0]
        self.assertEqual(
            field_type, connection.features.introspected_field_types["BooleanField"]
        )

    def test_add_field_default_transform(self):
        """
        Tests adding fields to models with a default that is not directly
        valid in the database (#22581)
        """

        class TestTransformField(IntegerField):
            # Weird field that saves the count of items in its value
            def get_default(self):
                return self.default

            def get_prep_value(self, value):
                if value is None:
                    return 0
                return len(value)

        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Add some rows of data
        Author.objects.create(name="Andrew", height=30)
        Author.objects.create(name="Andrea")
        # Add the field with a default it needs to cast (to string in this case)
        new_field = TestTransformField(default={1: 2})
        new_field.set_attributes_from_name("thing")
        with connection.schema_editor() as editor:
            editor.add_field(Author, new_field)
        # Ensure the field is there
        columns = self.column_classes(Author)
        field_type, field_info = columns["thing"]
        self.assertEqual(
            field_type, connection.features.introspected_field_types["IntegerField"]
        )
        # Make sure the values were transformed correctly
        self.assertEqual(Author.objects.extra(where=["thing = 1"]).count(), 2)

    def test_add_field_o2o_nullable(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Note)
        new_field = OneToOneField(Note, CASCADE, null=True)
        new_field.set_attributes_from_name("note")
        with connection.schema_editor() as editor:
            editor.add_field(Author, new_field)
        columns = self.column_classes(Author)
        self.assertIn("note_id", columns)
        self.assertTrue(columns["note_id"][1][6])

    def test_add_field_binary(self):
        """
        Tests binary fields get a sane default (#22851)
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Add the new field
        new_field = BinaryField(blank=True)
        new_field.set_attributes_from_name("bits")
        with connection.schema_editor() as editor:
            editor.add_field(Author, new_field)
        columns = self.column_classes(Author)
        # MySQL annoyingly uses the same backend, so it'll come back as one of
        # these two types.
        self.assertIn(columns["bits"][0], ("BinaryField", "TextField"))

    def test_add_field_durationfield_with_default(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        new_field = DurationField(default=datetime.timedelta(minutes=10))
        new_field.set_attributes_from_name("duration")
        with connection.schema_editor() as editor:
            editor.add_field(Author, new_field)
        columns = self.column_classes(Author)
        self.assertEqual(
            columns["duration"][0],
            connection.features.introspected_field_types["DurationField"],
        )

    @unittest.skipUnless(connection.vendor == "mysql", "MySQL specific")
    def test_add_binaryfield_mediumblob(self):
        """
        Test adding a custom-sized binary field on MySQL (#24846).
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Add the new field with default
        new_field = MediumBlobField(blank=True, default=b"123")
        new_field.set_attributes_from_name("bits")
        with connection.schema_editor() as editor:
            editor.add_field(Author, new_field)
        columns = self.column_classes(Author)
        # Introspection treats BLOBs as TextFields
        self.assertEqual(columns["bits"][0], "TextField")

    @isolate_apps("schema")
    @skipUnlessDBFeature("supports_json_field", "supports_stored_generated_columns")
    def test_add_generated_field_with_kt_model(self):
        class GeneratedFieldKTModel(Model):
            data = JSONField()
            status = GeneratedField(
                expression=KT("data__status"),
                output_field=TextField(),
                db_persist=True,
            )

            class Meta:
                app_label = "schema"

        with CaptureQueriesContext(connection) as ctx:
            with connection.schema_editor() as editor:
                editor.create_model(GeneratedFieldKTModel)
        self.assertIs(
            any("None" in query["sql"] for query in ctx.captured_queries),
            False,
        )

    @isolate_apps("schema")
    @skipUnlessDBFeature("supports_virtual_generated_columns")
    def test_add_generated_boolean_field(self):
        class GeneratedBooleanFieldModel(Model):
            value = IntegerField(null=True)
            has_value = GeneratedField(
                expression=Q(value__isnull=False),
                output_field=BooleanField(),
                db_persist=False,
            )

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(GeneratedBooleanFieldModel)
        obj = GeneratedBooleanFieldModel.objects.create()
        self.assertIs(obj.has_value, False)
        obj = GeneratedBooleanFieldModel.objects.create(value=1)
        self.assertIs(obj.has_value, True)

    @isolate_apps("schema")
    @skipUnlessDBFeature("supports_stored_generated_columns")
    def test_add_generated_field(self):
        class GeneratedFieldOutputFieldModel(Model):
            price = DecimalField(max_digits=7, decimal_places=2)
            vat_price = GeneratedField(
                expression=Round(F("price") * Value(Decimal("1.22")), 2),
                db_persist=True,
                output_field=DecimalField(max_digits=8, decimal_places=2),
            )

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(GeneratedFieldOutputFieldModel)

    @isolate_apps("schema")
    def test_add_auto_field(self):
        class AddAutoFieldModel(Model):
            name = CharField(max_length=255, primary_key=True)

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(AddAutoFieldModel)
        self.isolated_local_models = [AddAutoFieldModel]
        old_field = AddAutoFieldModel._meta.get_field("name")
        new_field = CharField(max_length=255)
        new_field.set_attributes_from_name("name")
        new_field.model = AddAutoFieldModel
        with connection.schema_editor() as editor:
            editor.alter_field(AddAutoFieldModel, old_field, new_field)
        new_auto_field = AutoField(primary_key=True)
        new_auto_field.set_attributes_from_name("id")
        new_auto_field.model = AddAutoFieldModel()
        with connection.schema_editor() as editor:
            editor.add_field(AddAutoFieldModel, new_auto_field)
        # Crashes on PostgreSQL when the GENERATED BY suffix is missing.
        AddAutoFieldModel.objects.create(name="test")

    def test_remove_field(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            with CaptureQueriesContext(connection) as ctx:
                editor.remove_field(Author, Author._meta.get_field("name"))
        columns = self.column_classes(Author)
        self.assertNotIn("name", columns)
        if getattr(connection.features, "can_alter_table_drop_column", True):
            # Table is not rebuilt.
            self.assertIs(
                any("CREATE TABLE" in query["sql"] for query in ctx.captured_queries),
                False,
            )
            self.assertIs(
                any("DROP TABLE" in query["sql"] for query in ctx.captured_queries),
                False,
            )

    def test_remove_indexed_field(self):
        with connection.schema_editor() as editor:
            editor.create_model(AuthorCharFieldWithIndex)
        with connection.schema_editor() as editor:
            editor.remove_field(
                AuthorCharFieldWithIndex,
                AuthorCharFieldWithIndex._meta.get_field("char_field"),
            )
        columns = self.column_classes(AuthorCharFieldWithIndex)
        self.assertNotIn("char_field", columns)

    def test_alter(self):
        """
        Tests simple altering of fields
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Ensure the field is right to begin with
        columns = self.column_classes(Author)
        self.assertEqual(
            columns["name"][0],
            connection.features.introspected_field_types["CharField"],
        )
        self.assertEqual(
            bool(columns["name"][1][6]),
            bool(connection.features.interprets_empty_strings_as_nulls),
        )
        # Alter the name field to a TextField
        old_field = Author._meta.get_field("name")
        new_field = TextField(null=True)
        new_field.set_attributes_from_name("name")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        columns = self.column_classes(Author)
        self.assertEqual(columns["name"][0], "TextField")
        self.assertTrue(columns["name"][1][6])
        # Change nullability again
        new_field2 = TextField(null=False)
        new_field2.set_attributes_from_name("name")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, new_field, new_field2, strict=True)
        columns = self.column_classes(Author)
        self.assertEqual(columns["name"][0], "TextField")
        self.assertEqual(
            bool(columns["name"][1][6]),
            bool(connection.features.interprets_empty_strings_as_nulls),
        )

    def test_alter_auto_field_to_integer_field(self):
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Change AutoField to IntegerField
        old_field = Author._meta.get_field("id")
        new_field = IntegerField(primary_key=True)
        new_field.set_attributes_from_name("id")
        new_field.model = Author
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        # Now that ID is an IntegerField, the database raises an error if it
        # isn't provided.
        if not connection.features.supports_unspecified_pk:
            with self.assertRaises(DatabaseError):
                Author.objects.create()

    def test_alter_auto_field_to_char_field(self):
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Change AutoField to CharField
        old_field = Author._meta.get_field("id")
        new_field = CharField(primary_key=True, max_length=50)
        new_field.set_attributes_from_name("id")
        new_field.model = Author
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)

    @isolate_apps("schema")
    def test_alter_auto_field_quoted_db_column(self):
        class Foo(Model):
            id = AutoField(primary_key=True, db_column='"quoted_id"')

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(Foo)
        self.isolated_local_models = [Foo]
        old_field = Foo._meta.get_field("id")
        new_field = BigAutoField(primary_key=True)
        new_field.model = Foo
        new_field.db_column = '"quoted_id"'
        new_field.set_attributes_from_name("id")
        with connection.schema_editor() as editor:
            editor.alter_field(Foo, old_field, new_field, strict=True)
        Foo.objects.create()

    def test_alter_not_unique_field_to_primary_key(self):
        # Create the table.
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Change UUIDField to primary key.
        old_field = Author._meta.get_field("uuid")
        new_field = UUIDField(primary_key=True)
        new_field.set_attributes_from_name("uuid")
        new_field.model = Author
        with connection.schema_editor() as editor:
            editor.remove_field(Author, Author._meta.get_field("id"))
            editor.alter_field(Author, old_field, new_field, strict=True)
        # Redundant unique constraint is not added.
        count = self.get_constraints_count(
            Author._meta.db_table,
            Author._meta.get_field("uuid").column,
            None,
        )
        self.assertLessEqual(count["uniques"], 1)

    @isolate_apps("schema")
    def test_alter_primary_key_quoted_db_table(self):
        class Foo(Model):
            class Meta:
                app_label = "schema"
                db_table = '"foo"'

        with connection.schema_editor() as editor:
            editor.create_model(Foo)
        self.isolated_local_models = [Foo]
        old_field = Foo._meta.get_field("id")
        new_field = BigAutoField(primary_key=True)
        new_field.model = Foo
        new_field.set_attributes_from_name("id")
        with connection.schema_editor() as editor:
            editor.alter_field(Foo, old_field, new_field, strict=True)
        Foo.objects.create()

    def test_alter_text_field(self):
        # Regression for "BLOB/TEXT column 'info' can't have a default value")
        # on MySQL.
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Note)
        old_field = Note._meta.get_field("info")
        new_field = TextField(blank=True)
        new_field.set_attributes_from_name("info")
        with connection.schema_editor() as editor:
            editor.alter_field(Note, old_field, new_field, strict=True)

    def test_alter_text_field_to_not_null_with_default_value(self):
        with connection.schema_editor() as editor:
            editor.create_model(Note)
        old_field = Note._meta.get_field("address")
        new_field = TextField(blank=True, default="", null=False)
        new_field.set_attributes_from_name("address")
        with connection.schema_editor() as editor:
            editor.alter_field(Note, old_field, new_field, strict=True)

    @skipUnlessDBFeature("can_defer_constraint_checks", "can_rollback_ddl")
    def test_alter_fk_checks_deferred_constraints(self):
        """
        #25492 - Altering a foreign key's structure and data in the same
        transaction.
        """
        with connection.schema_editor() as editor:
            editor.create_model(Node)
        old_field = Node._meta.get_field("parent")
        new_field = ForeignKey(Node, CASCADE)
        new_field.set_attributes_from_name("parent")
        parent = Node.objects.create()
        with connection.schema_editor() as editor:
            # Update the parent FK to create a deferred constraint check.
            Node.objects.update(parent=parent)
            editor.alter_field(Node, old_field, new_field, strict=True)

    @isolate_apps("schema")
    def test_alter_null_with_default_value_deferred_constraints(self):
        class Publisher(Model):
            class Meta:
                app_label = "schema"

        class Article(Model):
            publisher = ForeignKey(Publisher, CASCADE)
            title = CharField(max_length=50, null=True)
            description = CharField(max_length=100, null=True)

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(Publisher)
            editor.create_model(Article)
        self.isolated_local_models = [Article, Publisher]

        publisher = Publisher.objects.create()
        Article.objects.create(publisher=publisher)

        old_title = Article._meta.get_field("title")
        new_title = CharField(max_length=50, null=False, default="")
        new_title.set_attributes_from_name("title")
        old_description = Article._meta.get_field("description")
        new_description = CharField(max_length=100, null=False, default="")
        new_description.set_attributes_from_name("description")
        with connection.schema_editor() as editor:
            editor.alter_field(Article, old_title, new_title, strict=True)
            editor.alter_field(Article, old_description, new_description, strict=True)

    def test_alter_text_field_to_date_field(self):
        """
        #25002 - Test conversion of text field to date field.
        """
        with connection.schema_editor() as editor:
            editor.create_model(Note)
        Note.objects.create(info="1988-05-05")
        old_field = Note._meta.get_field("info")
        new_field = DateField(blank=True)
        new_field.set_attributes_from_name("info")
        with connection.schema_editor() as editor:
            editor.alter_field(Note, old_field, new_field, strict=True)
        # Make sure the field isn't nullable
        columns = self.column_classes(Note)
        self.assertFalse(columns["info"][1][6])

    def test_alter_text_field_to_datetime_field(self):
        """
        #25002 - Test conversion of text field to datetime field.
        """
        with connection.schema_editor() as editor:
            editor.create_model(Note)
        Note.objects.create(info="1988-05-05 3:16:17.4567")
        old_field = Note._meta.get_field("info")
        new_field = DateTimeField(blank=True)
        new_field.set_attributes_from_name("info")
        with connection.schema_editor() as editor:
            editor.alter_field(Note, old_field, new_field, strict=True)
        # Make sure the field isn't nullable
        columns = self.column_classes(Note)
        self.assertFalse(columns["info"][1][6])

    def test_alter_text_field_to_time_field(self):
        """
        #25002 - Test conversion of text field to time field.
        """
        with connection.schema_editor() as editor:
            editor.create_model(Note)
        Note.objects.create(info="3:16:17.4567")
        old_field = Note._meta.get_field("info")
        new_field = TimeField(blank=True)
        new_field.set_attributes_from_name("info")
        with connection.schema_editor() as editor:
            editor.alter_field(Note, old_field, new_field, strict=True)
        # Make sure the field isn't nullable
        columns = self.column_classes(Note)
        self.assertFalse(columns["info"][1][6])

    @skipIfDBFeature("interprets_empty_strings_as_nulls")
    def test_alter_textual_field_keep_null_status(self):
        """
        Changing a field type shouldn't affect the not null status.
        """
        with connection.schema_editor() as editor:
            editor.create_model(Note)
        with self.assertRaises(IntegrityError):
            Note.objects.create(info=None)
        old_field = Note._meta.get_field("info")
        new_field = CharField(max_length=50)
        new_field.set_attributes_from_name("info")
        with connection.schema_editor() as editor:
            editor.alter_field(Note, old_field, new_field, strict=True)
        with self.assertRaises(IntegrityError):
            Note.objects.create(info=None)

    @skipUnlessDBFeature("interprets_empty_strings_as_nulls")
    def test_alter_textual_field_not_null_to_null(self):
        """
        Nullability for textual fields is preserved on databases that
        interpret empty strings as NULLs.
        """
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        columns = self.column_classes(Author)
        # Field is nullable.
        self.assertTrue(columns["uuid"][1][6])
        # Change to NOT NULL.
        old_field = Author._meta.get_field("uuid")
        new_field = SlugField(null=False, blank=True)
        new_field.set_attributes_from_name("uuid")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        columns = self.column_classes(Author)
        # Nullability is preserved.
        self.assertTrue(columns["uuid"][1][6])

    def test_alter_numeric_field_keep_null_status(self):
        """
        Changing a field type shouldn't affect the not null status.
        """
        with connection.schema_editor() as editor:
            editor.create_model(UniqueTest)
        with self.assertRaises(IntegrityError):
            UniqueTest.objects.create(year=None, slug="aaa")
        old_field = UniqueTest._meta.get_field("year")
        new_field = BigIntegerField()
        new_field.set_attributes_from_name("year")
        with connection.schema_editor() as editor:
            editor.alter_field(UniqueTest, old_field, new_field, strict=True)
        with self.assertRaises(IntegrityError):
            UniqueTest.objects.create(year=None, slug="bbb")

    def test_alter_null_to_not_null(self):
        """
        #23609 - Tests handling of default values when altering from NULL to NOT NULL.
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Ensure the field is right to begin with
        columns = self.column_classes(Author)
        self.assertTrue(columns["height"][1][6])
        # Create some test data
        Author.objects.create(name="Not null author", height=12)
        Author.objects.create(name="Null author")
        # Verify null value
        self.assertEqual(Author.objects.get(name="Not null author").height, 12)
        self.assertIsNone(Author.objects.get(name="Null author").height)
        # Alter the height field to NOT NULL with default
        old_field = Author._meta.get_field("height")
        new_field = PositiveIntegerField(default=42)
        new_field.set_attributes_from_name("height")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        columns = self.column_classes(Author)
        self.assertFalse(columns["height"][1][6])
        # Verify default value
        self.assertEqual(Author.objects.get(name="Not null author").height, 12)
        self.assertEqual(Author.objects.get(name="Null author").height, 42)

    def test_alter_charfield_to_null(self):
        """
        #24307 - Should skip an alter statement on databases with
        interprets_empty_strings_as_nulls when changing a CharField to null.
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Change the CharField to null
        old_field = Author._meta.get_field("name")
        new_field = copy(old_field)
        new_field.null = True
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)

    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    def test_alter_char_field_decrease_length(self):
        # Create the table.
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        Author.objects.create(name="x" * 255)
        # Change max_length of CharField.
        old_field = Author._meta.get_field("name")
        new_field = CharField(max_length=254)
        new_field.set_attributes_from_name("name")
        with connection.schema_editor() as editor:
            msg = "value too long for type character varying(254)"
            with self.assertRaisesMessage(DataError, msg):
                editor.alter_field(Author, old_field, new_field, strict=True)

    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    def test_alter_field_with_custom_db_type(self):
        from django.contrib.postgres.fields import ArrayField

        class Foo(Model):
            field = ArrayField(CharField(max_length=255))

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(Foo)
        self.isolated_local_models = [Foo]
        old_field = Foo._meta.get_field("field")
        new_field = ArrayField(CharField(max_length=16))
        new_field.set_attributes_from_name("field")
        new_field.model = Foo
        with connection.schema_editor() as editor:
            editor.alter_field(Foo, old_field, new_field, strict=True)

    @isolate_apps("schema")
    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    def test_alter_array_field_decrease_base_field_length(self):
        from django.contrib.postgres.fields import ArrayField

        class ArrayModel(Model):
            field = ArrayField(CharField(max_length=16))

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(ArrayModel)
        self.isolated_local_models = [ArrayModel]
        ArrayModel.objects.create(field=["x" * 16])
        old_field = ArrayModel._meta.get_field("field")
        new_field = ArrayField(CharField(max_length=15))
        new_field.set_attributes_from_name("field")
        new_field.model = ArrayModel
        with connection.schema_editor() as editor:
            msg = "value too long for type character varying(15)"
            with self.assertRaisesMessage(DataError, msg):
                editor.alter_field(ArrayModel, old_field, new_field, strict=True)

    @isolate_apps("schema")
    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    def test_alter_array_field_decrease_nested_base_field_length(self):
        from django.contrib.postgres.fields import ArrayField

        class ArrayModel(Model):
            field = ArrayField(ArrayField(CharField(max_length=16)))

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(ArrayModel)
        self.isolated_local_models = [ArrayModel]
        ArrayModel.objects.create(field=[["x" * 16]])
        old_field = ArrayModel._meta.get_field("field")
        new_field = ArrayField(ArrayField(CharField(max_length=15)))
        new_field.set_attributes_from_name("field")
        new_field.model = ArrayModel
        with connection.schema_editor() as editor:
            msg = "value too long for type character varying(15)"
            with self.assertRaisesMessage(DataError, msg):
                editor.alter_field(ArrayModel, old_field, new_field, strict=True)

    def _add_ci_collation(self):
        ci_collation = "case_insensitive"

        def drop_collation():
            with connection.cursor() as cursor:
                cursor.execute(f"DROP COLLATION IF EXISTS {ci_collation}")

        with connection.cursor() as cursor:
            cursor.execute(
                f"CREATE COLLATION IF NOT EXISTS {ci_collation} (provider=icu, "
                f"locale='und-u-ks-level2', deterministic=false)"
            )
        self.addCleanup(drop_collation)
        return ci_collation

    @isolate_apps("schema")
    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    @skipUnlessDBFeature(
        "supports_collation_on_charfield",
        "supports_non_deterministic_collations",
    )
    def test_db_collation_arrayfield(self):
        from django.contrib.postgres.fields import ArrayField

        ci_collation = self._add_ci_collation()
        cs_collation = "en-x-icu"

        class ArrayModel(Model):
            field = ArrayField(CharField(max_length=16, db_collation=ci_collation))

            class Meta:
                app_label = "schema"

        # Create the table.
        with connection.schema_editor() as editor:
            editor.create_model(ArrayModel)
        self.isolated_local_models = [ArrayModel]
        self.assertEqual(
            self.get_column_collation(ArrayModel._meta.db_table, "field"),
            ci_collation,
        )
        # Alter collation.
        old_field = ArrayModel._meta.get_field("field")
        new_field_cs = ArrayField(CharField(max_length=16, db_collation=cs_collation))
        new_field_cs.set_attributes_from_name("field")
        new_field_cs.model = ArrayField
        with connection.schema_editor() as editor:
            editor.alter_field(ArrayModel, old_field, new_field_cs, strict=True)
        self.assertEqual(
            self.get_column_collation(ArrayModel._meta.db_table, "field"),
            cs_collation,
        )

    @isolate_apps("schema")
    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    @skipUnlessDBFeature(
        "supports_collation_on_charfield",
        "supports_non_deterministic_collations",
    )
    def test_unique_with_collation_charfield(self):
        ci_collation = self._add_ci_collation()

        class CiCharModel(Model):
            field = CharField(max_length=16, db_collation=ci_collation, unique=True)

            class Meta:
                app_label = "schema"

        # Create the table.
        with connection.schema_editor() as editor:
            editor.create_model(CiCharModel)
        self.isolated_local_models = [CiCharModel]
        self.assertEqual(
            self.get_column_collation(CiCharModel._meta.db_table, "field"),
            ci_collation,
        )
        self.assertIn("field", self.get_uniques(CiCharModel._meta.db_table))

    @isolate_apps("schema")
    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    @skipUnlessDBFeature("supports_collation_on_charfield")
    def test_unique_with_deterministic_collation_charfield(self):
        deterministic_collation = connection.features.test_collations.get(
            "deterministic"
        )
        if not deterministic_collation:
            self.skipTest("This backend does not support deterministic collations.")

        class CharModel(Model):
            field = CharField(db_collation=deterministic_collation, unique=True)

            class Meta:
                app_label = "schema"

        # Create the table.
        with connection.schema_editor() as editor:
            editor.create_model(CharModel)
        self.isolated_local_models = [CharModel]
        constraints = self.get_constraints_for_column(
            CharModel, CharModel._meta.get_field("field").column
        )
        self.assertIn("schema_charmodel_field_8b338dea_like", constraints)
        self.assertIn(
            "varchar_pattern_ops",
            self.get_constraint_opclasses("schema_charmodel_field_8b338dea_like"),
        )
        self.assertEqual(
            self.get_column_collation(CharModel._meta.db_table, "field"),
            deterministic_collation,
        )
        self.assertIn("field", self.get_uniques(CharModel._meta.db_table))

    @isolate_apps("schema")
    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    @skipUnlessDBFeature(
        "supports_collation_on_charfield",
        "supports_non_deterministic_collations",
    )
    def test_relation_to_collation_charfield(self):
        ci_collation = self._add_ci_collation()

        class CiCharModel(Model):
            field = CharField(max_length=16, db_collation=ci_collation, unique=True)

            class Meta:
                app_label = "schema"

        class RelationModel(Model):
            field = OneToOneField(CiCharModel, CASCADE, to_field="field")

            class Meta:
                app_label = "schema"

        # Create the table.
        with connection.schema_editor() as editor:
            editor.create_model(CiCharModel)
            editor.create_model(RelationModel)
        self.isolated_local_models = [CiCharModel, RelationModel]
        self.assertEqual(
            self.get_column_collation(RelationModel._meta.db_table, "field_id"),
            ci_collation,
        )
        self.assertEqual(
            self.get_column_collation(CiCharModel._meta.db_table, "field"),
            ci_collation,
        )
        self.assertIn("field_id", self.get_uniques(RelationModel._meta.db_table))

    @isolate_apps("schema")
    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    @skipUnlessDBFeature("supports_collation_on_charfield")
    def test_relation_to_deterministic_collation_charfield(self):
        deterministic_collation = connection.features.test_collations.get(
            "deterministic"
        )
        if not deterministic_collation:
            self.skipTest("This backend does not support deterministic collations.")

        class CharModel(Model):
            field = CharField(db_collation=deterministic_collation, unique=True)

            class Meta:
                app_label = "schema"

        class RelationModel(Model):
            field = OneToOneField(CharModel, CASCADE, to_field="field")

            class Meta:
                app_label = "schema"

        # Create the table.
        with connection.schema_editor() as editor:
            editor.create_model(CharModel)
            editor.create_model(RelationModel)
        self.isolated_local_models = [CharModel, RelationModel]
        constraints = self.get_constraints_for_column(
            CharModel, CharModel._meta.get_field("field").column
        )
        self.assertIn("schema_charmodel_field_8b338dea_like", constraints)
        self.assertIn(
            "varchar_pattern_ops",
            self.get_constraint_opclasses("schema_charmodel_field_8b338dea_like"),
        )
        rel_constraints = self.get_constraints_for_column(
            RelationModel, RelationModel._meta.get_field("field").column
        )
        self.assertIn("schema_relationmodel_field_id_395fbb08_like", rel_constraints)
        self.assertIn(
            "varchar_pattern_ops",
            self.get_constraint_opclasses(
                "schema_relationmodel_field_id_395fbb08_like"
            ),
        )
        self.assertEqual(
            self.get_column_collation(RelationModel._meta.db_table, "field_id"),
            deterministic_collation,
        )
        self.assertEqual(
            self.get_column_collation(CharModel._meta.db_table, "field"),
            deterministic_collation,
        )
        self.assertIn("field_id", self.get_uniques(RelationModel._meta.db_table))

    def test_alter_textfield_to_null(self):
        """
        #24307 - Should skip an alter statement on databases with
        interprets_empty_strings_as_nulls when changing a TextField to null.
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Note)
        # Change the TextField to null
        old_field = Note._meta.get_field("info")
        new_field = copy(old_field)
        new_field.null = True
        with connection.schema_editor() as editor:
            editor.alter_field(Note, old_field, new_field, strict=True)

    def test_alter_null_to_not_null_keeping_default(self):
        """
        #23738 - Can change a nullable field with default to non-nullable
        with the same default.
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(AuthorWithDefaultHeight)
        # Ensure the field is right to begin with
        columns = self.column_classes(AuthorWithDefaultHeight)
        self.assertTrue(columns["height"][1][6])
        # Alter the height field to NOT NULL keeping the previous default
        old_field = AuthorWithDefaultHeight._meta.get_field("height")
        new_field = PositiveIntegerField(default=42)
        new_field.set_attributes_from_name("height")
        with connection.schema_editor() as editor:
            editor.alter_field(
                AuthorWithDefaultHeight, old_field, new_field, strict=True
            )
        columns = self.column_classes(AuthorWithDefaultHeight)
        self.assertFalse(columns["height"][1][6])

    @skipUnlessDBFeature("supports_foreign_keys")
    def test_alter_fk(self):
        """
        Tests altering of FKs
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        # Ensure the field is right to begin with
        columns = self.column_classes(Book)
        self.assertEqual(
            columns["author_id"][0],
            connection.features.introspected_field_types["IntegerField"],
        )
        self.assertForeignKeyExists(Book, "author_id", "schema_author")
        # Alter the FK
        old_field = Book._meta.get_field("author")
        new_field = ForeignKey(Author, CASCADE, editable=False)
        new_field.set_attributes_from_name("author")
        with connection.schema_editor() as editor:
            editor.alter_field(Book, old_field, new_field, strict=True)
        columns = self.column_classes(Book)
        self.assertEqual(
            columns["author_id"][0],
            connection.features.introspected_field_types["IntegerField"],
        )
        self.assertForeignKeyExists(Book, "author_id", "schema_author")

    @skipUnlessDBFeature("supports_foreign_keys")
    def test_alter_to_fk(self):
        """
        #24447 - Tests adding a FK constraint for an existing column
        """

        class LocalBook(Model):
            author = IntegerField()
            title = CharField(max_length=100, db_index=True)
            pub_date = DateTimeField()

            class Meta:
                app_label = "schema"
                apps = new_apps

        self.local_models = [LocalBook]

        # Create the tables
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(LocalBook)
        # Ensure no FK constraint exists
        constraints = self.get_constraints(LocalBook._meta.db_table)
        for details in constraints.values():
            if details["foreign_key"]:
                self.fail(
                    "Found an unexpected FK constraint to %s" % details["columns"]
                )
        old_field = LocalBook._meta.get_field("author")
        new_field = ForeignKey(Author, CASCADE)
        new_field.set_attributes_from_name("author")
        with connection.schema_editor() as editor:
            editor.alter_field(LocalBook, old_field, new_field, strict=True)
        self.assertForeignKeyExists(LocalBook, "author_id", "schema_author")

    @skipUnlessDBFeature("supports_foreign_keys", "can_introspect_foreign_keys")
    def test_alter_o2o_to_fk(self):
        """
        #24163 - Tests altering of OneToOneField to ForeignKey
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(BookWithO2O)
        # Ensure the field is right to begin with
        columns = self.column_classes(BookWithO2O)
        self.assertEqual(
            columns["author_id"][0],
            connection.features.introspected_field_types["IntegerField"],
        )
        # Ensure the field is unique
        author = Author.objects.create(name="Joe")
        BookWithO2O.objects.create(
            author=author, title="Django 1", pub_date=datetime.datetime.now()
        )
        with self.assertRaises(IntegrityError):
            BookWithO2O.objects.create(
                author=author, title="Django 2", pub_date=datetime.datetime.now()
            )
        BookWithO2O.objects.all().delete()
        self.assertForeignKeyExists(BookWithO2O, "author_id", "schema_author")
        # Alter the OneToOneField to ForeignKey
        old_field = BookWithO2O._meta.get_field("author")
        new_field = ForeignKey(Author, CASCADE)
        new_field.set_attributes_from_name("author")
        with connection.schema_editor() as editor:
            editor.alter_field(BookWithO2O, old_field, new_field, strict=True)
        columns = self.column_classes(Book)
        self.assertEqual(
            columns["author_id"][0],
            connection.features.introspected_field_types["IntegerField"],
        )
        # Ensure the field is not unique anymore
        Book.objects.create(
            author=author, title="Django 1", pub_date=datetime.datetime.now()
        )
        Book.objects.create(
            author=author, title="Django 2", pub_date=datetime.datetime.now()
        )
        self.assertForeignKeyExists(Book, "author_id", "schema_author")

    @skipUnlessDBFeature("supports_foreign_keys", "can_introspect_foreign_keys")
    def test_alter_fk_to_o2o(self):
        """
        #24163 - Tests altering of ForeignKey to OneToOneField
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        # Ensure the field is right to begin with
        columns = self.column_classes(Book)
        self.assertEqual(
            columns["author_id"][0],
            connection.features.introspected_field_types["IntegerField"],
        )
        # Ensure the field is not unique
        author = Author.objects.create(name="Joe")
        Book.objects.create(
            author=author, title="Django 1", pub_date=datetime.datetime.now()
        )
        Book.objects.create(
            author=author, title="Django 2", pub_date=datetime.datetime.now()
        )
        Book.objects.all().delete()
        self.assertForeignKeyExists(Book, "author_id", "schema_author")
        # Alter the ForeignKey to OneToOneField
        old_field = Book._meta.get_field("author")
        new_field = OneToOneField(Author, CASCADE)
        new_field.set_attributes_from_name("author")
        with connection.schema_editor() as editor:
            editor.alter_field(Book, old_field, new_field, strict=True)
        columns = self.column_classes(BookWithO2O)
        self.assertEqual(
            columns["author_id"][0],
            connection.features.introspected_field_types["IntegerField"],
        )
        # Ensure the field is unique now
        BookWithO2O.objects.create(
            author=author, title="Django 1", pub_date=datetime.datetime.now()
        )
        with self.assertRaises(IntegrityError):
            BookWithO2O.objects.create(
                author=author, title="Django 2", pub_date=datetime.datetime.now()
            )
        self.assertForeignKeyExists(BookWithO2O, "author_id", "schema_author")

    def test_alter_field_fk_to_o2o(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        expected_fks = (
            1
            if connection.features.supports_foreign_keys
            and connection.features.can_introspect_foreign_keys
            else 0
        )
        expected_indexes = 1 if connection.features.indexes_foreign_keys else 0

        # Check the index is right to begin with.
        counts = self.get_constraints_count(
            Book._meta.db_table,
            Book._meta.get_field("author").column,
            (Author._meta.db_table, Author._meta.pk.column),
        )
        self.assertEqual(
            counts,
            {"fks": expected_fks, "uniques": 0, "indexes": expected_indexes},
        )

        old_field = Book._meta.get_field("author")
        new_field = OneToOneField(Author, CASCADE)
        new_field.set_attributes_from_name("author")
        with connection.schema_editor() as editor:
            editor.alter_field(Book, old_field, new_field)

        counts = self.get_constraints_count(
            Book._meta.db_table,
            Book._meta.get_field("author").column,
            (Author._meta.db_table, Author._meta.pk.column),
        )
        # The index on ForeignKey is replaced with a unique constraint for
        # OneToOneField.
        self.assertEqual(counts, {"fks": expected_fks, "uniques": 1, "indexes": 0})

    def test_autofield_to_o2o(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Note)

        # Rename the field.
        old_field = Author._meta.get_field("id")
        new_field = AutoField(primary_key=True)
        new_field.set_attributes_from_name("note_ptr")
        new_field.model = Author

        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        # Alter AutoField to OneToOneField.
        new_field_o2o = OneToOneField(Note, CASCADE)
        new_field_o2o.set_attributes_from_name("note_ptr")
        new_field_o2o.model = Author

        with connection.schema_editor() as editor:
            editor.alter_field(Author, new_field, new_field_o2o, strict=True)
        columns = self.column_classes(Author)
        field_type, _ = columns["note_ptr_id"]
        self.assertEqual(
            field_type, connection.features.introspected_field_types["IntegerField"]
        )

    def test_alter_field_fk_keeps_index(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        expected_fks = (
            1
            if connection.features.supports_foreign_keys
            and connection.features.can_introspect_foreign_keys
            else 0
        )
        expected_indexes = 1 if connection.features.indexes_foreign_keys else 0

        # Check the index is right to begin with.
        counts = self.get_constraints_count(
            Book._meta.db_table,
            Book._meta.get_field("author").column,
            (Author._meta.db_table, Author._meta.pk.column),
        )
        self.assertEqual(
            counts,
            {"fks": expected_fks, "uniques": 0, "indexes": expected_indexes},
        )

        old_field = Book._meta.get_field("author")
        # on_delete changed from CASCADE.
        new_field = ForeignKey(Author, PROTECT)
        new_field.set_attributes_from_name("author")
        with connection.schema_editor() as editor:
            editor.alter_field(Book, old_field, new_field, strict=True)

        counts = self.get_constraints_count(
            Book._meta.db_table,
            Book._meta.get_field("author").column,
            (Author._meta.db_table, Author._meta.pk.column),
        )
        # The index remains.
        self.assertEqual(
            counts,
            {"fks": expected_fks, "uniques": 0, "indexes": expected_indexes},
        )

    def test_alter_field_o2o_to_fk(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(BookWithO2O)
        expected_fks = (
            1
            if connection.features.supports_foreign_keys
            and connection.features.can_introspect_foreign_keys
            else 0
        )

        # Check the unique constraint is right to begin with.
        counts = self.get_constraints_count(
            BookWithO2O._meta.db_table,
            BookWithO2O._meta.get_field("author").column,
            (Author._meta.db_table, Author._meta.pk.column),
        )
        self.assertEqual(counts, {"fks": expected_fks, "uniques": 1, "indexes": 0})

        old_field = BookWithO2O._meta.get_field("author")
        new_field = ForeignKey(Author, CASCADE)
        new_field.set_attributes_from_name("author")
        with connection.schema_editor() as editor:
            editor.alter_field(BookWithO2O, old_field, new_field)

        counts = self.get_constraints_count(
            BookWithO2O._meta.db_table,
            BookWithO2O._meta.get_field("author").column,
            (Author._meta.db_table, Author._meta.pk.column),
        )
        # The unique constraint on OneToOneField is replaced with an index for
        # ForeignKey.
        self.assertEqual(counts, {"fks": expected_fks, "uniques": 0, "indexes": 1})

    def test_alter_field_o2o_keeps_unique(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(BookWithO2O)
        expected_fks = (
            1
            if connection.features.supports_foreign_keys
            and connection.features.can_introspect_foreign_keys
            else 0
        )

        # Check the unique constraint is right to begin with.
        counts = self.get_constraints_count(
            BookWithO2O._meta.db_table,
            BookWithO2O._meta.get_field("author").column,
            (Author._meta.db_table, Author._meta.pk.column),
        )
        self.assertEqual(counts, {"fks": expected_fks, "uniques": 1, "indexes": 0})

        old_field = BookWithO2O._meta.get_field("author")
        # on_delete changed from CASCADE.
        new_field = OneToOneField(Author, PROTECT)
        new_field.set_attributes_from_name("author")
        with connection.schema_editor() as editor:
            editor.alter_field(BookWithO2O, old_field, new_field, strict=True)

        counts = self.get_constraints_count(
            BookWithO2O._meta.db_table,
            BookWithO2O._meta.get_field("author").column,
            (Author._meta.db_table, Author._meta.pk.column),
        )
        # The unique constraint remains.
        self.assertEqual(counts, {"fks": expected_fks, "uniques": 1, "indexes": 0})

    @skipUnlessDBFeature("ignores_table_name_case")
    def test_alter_db_table_case(self):
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Alter the case of the table
        old_table_name = Author._meta.db_table
        with connection.schema_editor() as editor:
            editor.alter_db_table(Author, old_table_name, old_table_name.upper())

    def test_alter_implicit_id_to_explicit(self):
        """
        Should be able to convert an implicit "id" field to an explicit "id"
        primary key field.
        """
        with connection.schema_editor() as editor:
            editor.create_model(Author)

        old_field = Author._meta.get_field("id")
        new_field = AutoField(primary_key=True)
        new_field.set_attributes_from_name("id")
        new_field.model = Author
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        # This will fail if DROP DEFAULT is inadvertently executed on this
        # field which drops the id sequence, at least on PostgreSQL.
        Author.objects.create(name="Foo")
        Author.objects.create(name="Bar")

    def test_alter_autofield_pk_to_bigautofield_pk(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        old_field = Author._meta.get_field("id")
        new_field = BigAutoField(primary_key=True)
        new_field.set_attributes_from_name("id")
        new_field.model = Author
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)

        Author.objects.create(name="Foo", pk=1)
        with connection.cursor() as cursor:
            sequence_reset_sqls = connection.ops.sequence_reset_sql(
                no_style(), [Author]
            )
            if sequence_reset_sqls:
                cursor.execute(sequence_reset_sqls[0])
        self.assertIsNotNone(Author.objects.create(name="Bar"))

    def test_alter_autofield_pk_to_smallautofield_pk(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        old_field = Author._meta.get_field("id")
        new_field = SmallAutoField(primary_key=True)
        new_field.set_attributes_from_name("id")
        new_field.model = Author
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)

        Author.objects.create(name="Foo", pk=1)
        with connection.cursor() as cursor:
            sequence_reset_sqls = connection.ops.sequence_reset_sql(
                no_style(), [Author]
            )
            if sequence_reset_sqls:
                cursor.execute(sequence_reset_sqls[0])
        self.assertIsNotNone(Author.objects.create(name="Bar"))

    def test_alter_int_pk_to_autofield_pk(self):
        """
        Should be able to rename an IntegerField(primary_key=True) to
        AutoField(primary_key=True).
        """
        with connection.schema_editor() as editor:
            editor.create_model(IntegerPK)

        old_field = IntegerPK._meta.get_field("i")
        new_field = AutoField(primary_key=True)
        new_field.model = IntegerPK
        new_field.set_attributes_from_name("i")

        with connection.schema_editor() as editor:
            editor.alter_field(IntegerPK, old_field, new_field, strict=True)

        # A model representing the updated model.
        class IntegerPKToAutoField(Model):
            i = AutoField(primary_key=True)
            j = IntegerField(unique=True)

            class Meta:
                app_label = "schema"
                apps = new_apps
                db_table = IntegerPK._meta.db_table

        # An id (i) is generated by the database.
        obj = IntegerPKToAutoField.objects.create(j=1)
        self.assertIsNotNone(obj.i)

    def test_alter_int_pk_to_bigautofield_pk(self):
        """
        Should be able to rename an IntegerField(primary_key=True) to
        BigAutoField(primary_key=True).
        """
        with connection.schema_editor() as editor:
            editor.create_model(IntegerPK)

        old_field = IntegerPK._meta.get_field("i")
        new_field = BigAutoField(primary_key=True)
        new_field.model = IntegerPK
        new_field.set_attributes_from_name("i")

        with connection.schema_editor() as editor:
            editor.alter_field(IntegerPK, old_field, new_field, strict=True)

        # A model representing the updated model.
        class IntegerPKToBigAutoField(Model):
            i = BigAutoField(primary_key=True)
            j = IntegerField(unique=True)

            class Meta:
                app_label = "schema"
                apps = new_apps
                db_table = IntegerPK._meta.db_table

        # An id (i) is generated by the database.
        obj = IntegerPKToBigAutoField.objects.create(j=1)
        self.assertIsNotNone(obj.i)

    @isolate_apps("schema")
    def test_alter_smallint_pk_to_smallautofield_pk(self):
        """
        Should be able to rename an SmallIntegerField(primary_key=True) to
        SmallAutoField(primary_key=True).
        """

        class SmallIntegerPK(Model):
            i = SmallIntegerField(primary_key=True)

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(SmallIntegerPK)
        self.isolated_local_models = [SmallIntegerPK]
        old_field = SmallIntegerPK._meta.get_field("i")
        new_field = SmallAutoField(primary_key=True)
        new_field.model = SmallIntegerPK
        new_field.set_attributes_from_name("i")
        with connection.schema_editor() as editor:
            editor.alter_field(SmallIntegerPK, old_field, new_field, strict=True)

    @isolate_apps("schema")
    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    def test_alter_serial_auto_field_to_bigautofield(self):
        class SerialAutoField(Model):
            id = SmallAutoField(primary_key=True)

            class Meta:
                app_label = "schema"

        table = SerialAutoField._meta.db_table
        column = SerialAutoField._meta.get_field("id").column
        with connection.cursor() as cursor:
            cursor.execute(
                f'CREATE TABLE "{table}" '
                f'("{column}" smallserial NOT NULL PRIMARY KEY)'
            )
        try:
            old_field = SerialAutoField._meta.get_field("id")
            new_field = BigAutoField(primary_key=True)
            new_field.model = SerialAutoField
            new_field.set_attributes_from_name("id")
            with connection.schema_editor() as editor:
                editor.alter_field(SerialAutoField, old_field, new_field, strict=True)
            sequence_name = f"{table}_{column}_seq"
            with connection.cursor() as cursor:
                cursor.execute(
                    "SELECT data_type FROM pg_sequences WHERE sequencename = %s",
                    [sequence_name],
                )
                row = cursor.fetchone()
                sequence_data_type = row[0] if row and row[0] else None
                self.assertEqual(sequence_data_type, "bigint")
            # Rename the column.
            old_field = new_field
            new_field = AutoField(primary_key=True)
            new_field.model = SerialAutoField
            new_field.set_attributes_from_name("renamed_id")
            with connection.schema_editor() as editor:
                editor.alter_field(SerialAutoField, old_field, new_field, strict=True)
            with connection.cursor() as cursor:
                cursor.execute(
                    "SELECT data_type FROM pg_sequences WHERE sequencename = %s",
                    [sequence_name],
                )
                row = cursor.fetchone()
                sequence_data_type = row[0] if row and row[0] else None
                self.assertEqual(sequence_data_type, "integer")
        finally:
            with connection.cursor() as cursor:
                cursor.execute(f'DROP TABLE "{table}"')

    def test_alter_int_pk_to_int_unique(self):
        """
        Should be able to rename an IntegerField(primary_key=True) to
        IntegerField(unique=True).
        """
        with connection.schema_editor() as editor:
            editor.create_model(IntegerPK)
        # Delete the old PK
        old_field = IntegerPK._meta.get_field("i")
        new_field = IntegerField(unique=True)
        new_field.model = IntegerPK
        new_field.set_attributes_from_name("i")
        with connection.schema_editor() as editor:
            editor.alter_field(IntegerPK, old_field, new_field, strict=True)
        # The primary key constraint is gone. Result depends on database:
        # 'id' for SQLite, None for others (must not be 'i').
        self.assertIn(self.get_primary_key(IntegerPK._meta.db_table), ("id", None))

        # Set up a model class as it currently stands. The original IntegerPK
        # class is now out of date and some backends make use of the whole
        # model class when modifying a field (such as sqlite3 when remaking a
        # table) so an outdated model class leads to incorrect results.
        class Transitional(Model):
            i = IntegerField(unique=True)
            j = IntegerField(unique=True)

            class Meta:
                app_label = "schema"
                apps = new_apps
                db_table = "INTEGERPK"

        # model requires a new PK
        old_field = Transitional._meta.get_field("j")
        new_field = IntegerField(primary_key=True)
        new_field.model = Transitional
        new_field.set_attributes_from_name("j")

        with connection.schema_editor() as editor:
            editor.alter_field(Transitional, old_field, new_field, strict=True)

        # Create a model class representing the updated model.
        class IntegerUnique(Model):
            i = IntegerField(unique=True)
            j = IntegerField(primary_key=True)

            class Meta:
                app_label = "schema"
                apps = new_apps
                db_table = "INTEGERPK"

        # Ensure unique constraint works.
        IntegerUnique.objects.create(i=1, j=1)
        with self.assertRaises(IntegrityError):
            IntegerUnique.objects.create(i=1, j=2)

    def test_rename(self):
        """
        Tests simple altering of fields
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Ensure the field is right to begin with
        columns = self.column_classes(Author)
        self.assertEqual(
            columns["name"][0],
            connection.features.introspected_field_types["CharField"],
        )
        self.assertNotIn("display_name", columns)
        # Alter the name field's name
        old_field = Author._meta.get_field("name")
        new_field = CharField(max_length=254)
        new_field.set_attributes_from_name("display_name")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        columns = self.column_classes(Author)
        self.assertEqual(
            columns["display_name"][0],
            connection.features.introspected_field_types["CharField"],
        )
        self.assertNotIn("name", columns)

    @isolate_apps("schema")
    def test_rename_referenced_field(self):
        class Author(Model):
            name = CharField(max_length=255, unique=True)

            class Meta:
                app_label = "schema"

        class Book(Model):
            author = ForeignKey(Author, CASCADE, to_field="name")

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        new_field = CharField(max_length=255, unique=True)
        new_field.set_attributes_from_name("renamed")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, Author._meta.get_field("name"), new_field)
        # Ensure the foreign key reference was updated.
        self.assertForeignKeyExists(Book, "author_id", "schema_author", "renamed")

    @skipIfDBFeature("interprets_empty_strings_as_nulls")
    def test_rename_keep_null_status(self):
        """
        Renaming a field shouldn't affect the not null status.
        """
        with connection.schema_editor() as editor:
            editor.create_model(Note)
        with self.assertRaises(IntegrityError):
            Note.objects.create(info=None)
        old_field = Note._meta.get_field("info")
        new_field = TextField()
        new_field.set_attributes_from_name("detail_info")
        with connection.schema_editor() as editor:
            editor.alter_field(Note, old_field, new_field, strict=True)
        columns = self.column_classes(Note)
        self.assertEqual(columns["detail_info"][0], "TextField")
        self.assertNotIn("info", columns)
        with self.assertRaises(IntegrityError):
            NoteRename.objects.create(detail_info=None)

    @isolate_apps("schema")
    def test_rename_keep_db_default(self):
        """Renaming a field shouldn't affect a database default."""

        class AuthorDbDefault(Model):
            birth_year = IntegerField(db_default=1985)

            class Meta:
                app_label = "schema"

        self.isolated_local_models = [AuthorDbDefault]
        with connection.schema_editor() as editor:
            editor.create_model(AuthorDbDefault)
        columns = self.column_classes(AuthorDbDefault)
        self.assertEqual(columns["birth_year"][1].default, "1985")

        old_field = AuthorDbDefault._meta.get_field("birth_year")
        new_field = IntegerField(db_default=1985)
        new_field.set_attributes_from_name("renamed_year")
        new_field.model = AuthorDbDefault
        with connection.schema_editor() as editor:
            editor.alter_field(AuthorDbDefault, old_field, new_field, strict=True)
        columns = self.column_classes(AuthorDbDefault)
        self.assertEqual(columns["renamed_year"][1].default, "1985")

    @isolate_apps("schema")
    def test_add_field_both_defaults_preserves_db_default(self):
        class Author(Model):
            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(Author)

        field = IntegerField(default=1985, db_default=1988)
        field.set_attributes_from_name("birth_year")
        field.model = Author
        with connection.schema_editor() as editor:
            editor.add_field(Author, field)
        columns = self.column_classes(Author)
        self.assertEqual(columns["birth_year"][1].default, "1988")

    @isolate_apps("schema")
    def test_add_text_field_with_db_default(self):
        class Author(Model):
            description = TextField(db_default="(missing)")

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(Author)
        columns = self.column_classes(Author)
        self.assertIn("(missing)", columns["description"][1].default)

    @isolate_apps("schema")
    def test_db_default_equivalent_sql_noop(self):
        class Author(Model):
            name = TextField(db_default=Value("foo"))

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(Author)

        new_field = TextField(db_default="foo")
        new_field.set_attributes_from_name("name")
        new_field.model = Author
        with connection.schema_editor() as editor, self.assertNumQueries(0):
            editor.alter_field(Author, Author._meta.get_field("name"), new_field)

    @isolate_apps("schema")
    def test_db_default_output_field_resolving(self):
        class Author(Model):
            data = JSONField(
                encoder=DjangoJSONEncoder,
                db_default={
                    "epoch": datetime.datetime(1970, 1, 1, tzinfo=datetime.timezone.utc)
                },
            )

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(Author)

        author = Author.objects.create()
        author.refresh_from_db()
        self.assertEqual(author.data, {"epoch": "1970-01-01T00:00:00Z"})

    @skipUnlessDBFeature(
        "supports_column_check_constraints", "can_introspect_check_constraints"
    )
    @isolate_apps("schema")
    def test_rename_field_with_check_to_truncated_name(self):
        class AuthorWithLongColumn(Model):
            field_with_very_looooooong_name = PositiveIntegerField(null=True)

            class Meta:
                app_label = "schema"

        self.isolated_local_models = [AuthorWithLongColumn]
        with connection.schema_editor() as editor:
            editor.create_model(AuthorWithLongColumn)
        old_field = AuthorWithLongColumn._meta.get_field(
            "field_with_very_looooooong_name"
        )
        new_field = PositiveIntegerField(null=True)
        new_field.set_attributes_from_name("renamed_field_with_very_long_name")
        with connection.schema_editor() as editor:
            editor.alter_field(AuthorWithLongColumn, old_field, new_field, strict=True)

        new_field_name = truncate_name(
            new_field.column, connection.ops.max_name_length()
        )
        constraints = self.get_constraints(AuthorWithLongColumn._meta.db_table)
        check_constraints = [
            name
            for name, details in constraints.items()
            if details["columns"] == [new_field_name] and details["check"]
        ]
        self.assertEqual(len(check_constraints), 1)

    def _test_m2m_create(self, M2MFieldClass):
        """
        Tests M2M fields on models during creation
        """

        class LocalBookWithM2M(Model):
            author = ForeignKey(Author, CASCADE)
            title = CharField(max_length=100, db_index=True)
            pub_date = DateTimeField()
            tags = M2MFieldClass("TagM2MTest", related_name="books")

            class Meta:
                app_label = "schema"
                apps = new_apps

        self.local_models = [LocalBookWithM2M]
        # Create the tables
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(TagM2MTest)
            editor.create_model(LocalBookWithM2M)
        # Ensure there is now an m2m table there
        columns = self.column_classes(
            LocalBookWithM2M._meta.get_field("tags").remote_field.through
        )
        self.assertEqual(
            columns["tagm2mtest_id"][0],
            connection.features.introspected_field_types["IntegerField"],
        )

    def test_m2m_create(self):
        self._test_m2m_create(ManyToManyField)

    def test_m2m_create_custom(self):
        self._test_m2m_create(CustomManyToManyField)

    def test_m2m_create_inherited(self):
        self._test_m2m_create(InheritedManyToManyField)

    def _test_m2m_create_through(self, M2MFieldClass):
        """
        Tests M2M fields on models during creation with through models
        """

        class LocalTagThrough(Model):
            book = ForeignKey("schema.LocalBookWithM2MThrough", CASCADE)
            tag = ForeignKey("schema.TagM2MTest", CASCADE)

            class Meta:
                app_label = "schema"
                apps = new_apps

        class LocalBookWithM2MThrough(Model):
            tags = M2MFieldClass(
                "TagM2MTest", related_name="books", through=LocalTagThrough
            )

            class Meta:
                app_label = "schema"
                apps = new_apps

        self.local_models = [LocalTagThrough, LocalBookWithM2MThrough]

        # Create the tables
        with connection.schema_editor() as editor:
            editor.create_model(LocalTagThrough)
            editor.create_model(TagM2MTest)
            editor.create_model(LocalBookWithM2MThrough)
        # Ensure there is now an m2m table there
        columns = self.column_classes(LocalTagThrough)
        self.assertEqual(
            columns["book_id"][0],
            connection.features.introspected_field_types["IntegerField"],
        )
        self.assertEqual(
            columns["tag_id"][0],
            connection.features.introspected_field_types["IntegerField"],
        )

    def test_m2m_create_through(self):
        self._test_m2m_create_through(ManyToManyField)

    def test_m2m_create_through_custom(self):
        self._test_m2m_create_through(CustomManyToManyField)

    def test_m2m_create_through_inherited(self):
        self._test_m2m_create_through(InheritedManyToManyField)

    def test_m2m_through_remove(self):
        class LocalAuthorNoteThrough(Model):
            book = ForeignKey("schema.Author", CASCADE)
            tag = ForeignKey("self", CASCADE)

            class Meta:
                app_label = "schema"
                apps = new_apps

        class LocalNoteWithM2MThrough(Model):
            authors = ManyToManyField("schema.Author", through=LocalAuthorNoteThrough)

            class Meta:
                app_label = "schema"
                apps = new_apps

        self.local_models = [LocalAuthorNoteThrough, LocalNoteWithM2MThrough]
        # Create the tables.
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(LocalAuthorNoteThrough)
            editor.create_model(LocalNoteWithM2MThrough)
        # Remove the through parameter.
        old_field = LocalNoteWithM2MThrough._meta.get_field("authors")
        new_field = ManyToManyField("Author")
        new_field.set_attributes_from_name("authors")
        msg = (
            f"Cannot alter field {old_field} into {new_field} - they are not "
            f"compatible types (you cannot alter to or from M2M fields, or add or "
            f"remove through= on M2M fields)"
        )
        with connection.schema_editor() as editor:
            with self.assertRaisesMessage(ValueError, msg):
                editor.alter_field(LocalNoteWithM2MThrough, old_field, new_field)

    def _test_m2m(self, M2MFieldClass):
        """
        Tests adding/removing M2M fields on models
        """

        class LocalAuthorWithM2M(Model):
            name = CharField(max_length=255)

            class Meta:
                app_label = "schema"
                apps = new_apps

        self.local_models = [LocalAuthorWithM2M]

        # Create the tables
        with connection.schema_editor() as editor:
            editor.create_model(LocalAuthorWithM2M)
            editor.create_model(TagM2MTest)
        # Create an M2M field
        new_field = M2MFieldClass("schema.TagM2MTest", related_name="authors")
        new_field.contribute_to_class(LocalAuthorWithM2M, "tags")
        # Ensure there's no m2m table there
        with self.assertRaises(DatabaseError):
            self.column_classes(new_field.remote_field.through)
        # Add the field
        with (
            CaptureQueriesContext(connection) as ctx,
            connection.schema_editor() as editor,
        ):
            editor.add_field(LocalAuthorWithM2M, new_field)
        # Table is not rebuilt.
        self.assertEqual(
            len(
                [
                    query["sql"]
                    for query in ctx.captured_queries
                    if "CREATE TABLE" in query["sql"]
                ]
            ),
            1,
        )
        self.assertIs(
            any("DROP TABLE" in query["sql"] for query in ctx.captured_queries),
            False,
        )
        # Ensure there is now an m2m table there
        columns = self.column_classes(new_field.remote_field.through)
        self.assertEqual(
            columns["tagm2mtest_id"][0],
            connection.features.introspected_field_types["IntegerField"],
        )

        # "Alter" the field. This should not rename the DB table to itself.
        with connection.schema_editor() as editor:
            editor.alter_field(LocalAuthorWithM2M, new_field, new_field, strict=True)

        # Remove the M2M table again
        with connection.schema_editor() as editor:
            editor.remove_field(LocalAuthorWithM2M, new_field)
        # Ensure there's no m2m table there
        with self.assertRaises(DatabaseError):
            self.column_classes(new_field.remote_field.through)

        # Make sure the model state is coherent with the table one now that
        # we've removed the tags field.
        opts = LocalAuthorWithM2M._meta
        opts.local_many_to_many.remove(new_field)
        del new_apps.all_models["schema"][
            new_field.remote_field.through._meta.model_name
        ]
        opts._expire_cache()

    def test_m2m(self):
        self._test_m2m(ManyToManyField)

    def test_m2m_custom(self):
        self._test_m2m(CustomManyToManyField)

    def test_m2m_inherited(self):
        self._test_m2m(InheritedManyToManyField)

    def _test_m2m_through_alter(self, M2MFieldClass):
        """
        Tests altering M2Ms with explicit through models (should no-op)
        """

        class LocalAuthorTag(Model):
            author = ForeignKey("schema.LocalAuthorWithM2MThrough", CASCADE)
            tag = ForeignKey("schema.TagM2MTest", CASCADE)

            class Meta:
                app_label = "schema"
                apps = new_apps

        class LocalAuthorWithM2MThrough(Model):
            name = CharField(max_length=255)
            tags = M2MFieldClass(
                "schema.TagM2MTest", related_name="authors", through=LocalAuthorTag
            )

            class Meta:
                app_label = "schema"
                apps = new_apps

        self.local_models = [LocalAuthorTag, LocalAuthorWithM2MThrough]

        # Create the tables
        with connection.schema_editor() as editor:
            editor.create_model(LocalAuthorTag)
            editor.create_model(LocalAuthorWithM2MThrough)
            editor.create_model(TagM2MTest)
        # Ensure the m2m table is there
        self.assertEqual(len(self.column_classes(LocalAuthorTag)), 3)
        # "Alter" the field's blankness. This should not actually do anything.
        old_field = LocalAuthorWithM2MThrough._meta.get_field("tags")
        new_field = M2MFieldClass(
            "schema.TagM2MTest", related_name="authors", through=LocalAuthorTag
        )
        new_field.contribute_to_class(LocalAuthorWithM2MThrough, "tags")
        with connection.schema_editor() as editor:
            editor.alter_field(
                LocalAuthorWithM2MThrough, old_field, new_field, strict=True
            )
        # Ensure the m2m table is still there
        self.assertEqual(len(self.column_classes(LocalAuthorTag)), 3)

    def test_m2m_through_alter(self):
        self._test_m2m_through_alter(ManyToManyField)

    def test_m2m_through_alter_custom(self):
        self._test_m2m_through_alter(CustomManyToManyField)

    def test_m2m_through_alter_inherited(self):
        self._test_m2m_through_alter(InheritedManyToManyField)

    def _test_m2m_repoint(self, M2MFieldClass):
        """
        Tests repointing M2M fields
        """

        class LocalBookWithM2M(Model):
            author = ForeignKey(Author, CASCADE)
            title = CharField(max_length=100, db_index=True)
            pub_date = DateTimeField()
            tags = M2MFieldClass("TagM2MTest", related_name="books")

            class Meta:
                app_label = "schema"
                apps = new_apps

        self.local_models = [LocalBookWithM2M]
        # Create the tables
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(LocalBookWithM2M)
            editor.create_model(TagM2MTest)
            editor.create_model(UniqueTest)
        # Ensure the M2M exists and points to TagM2MTest
        if connection.features.supports_foreign_keys:
            self.assertForeignKeyExists(
                LocalBookWithM2M._meta.get_field("tags").remote_field.through,
                "tagm2mtest_id",
                "schema_tagm2mtest",
            )
        # Repoint the M2M
        old_field = LocalBookWithM2M._meta.get_field("tags")
        new_field = M2MFieldClass(UniqueTest)
        new_field.contribute_to_class(LocalBookWithM2M, "uniques")
        with connection.schema_editor() as editor:
            editor.alter_field(LocalBookWithM2M, old_field, new_field, strict=True)
        # Ensure old M2M is gone
        with self.assertRaises(DatabaseError):
            self.column_classes(
                LocalBookWithM2M._meta.get_field("tags").remote_field.through
            )

        # This model looks like the new model and is used for teardown.
        opts = LocalBookWithM2M._meta
        opts.local_many_to_many.remove(old_field)
        # Ensure the new M2M exists and points to UniqueTest
        if connection.features.supports_foreign_keys:
            self.assertForeignKeyExists(
                new_field.remote_field.through, "uniquetest_id", "schema_uniquetest"
            )

    def test_m2m_repoint(self):
        self._test_m2m_repoint(ManyToManyField)

    def test_m2m_repoint_custom(self):
        self._test_m2m_repoint(CustomManyToManyField)

    def test_m2m_repoint_inherited(self):
        self._test_m2m_repoint(InheritedManyToManyField)

    @isolate_apps("schema")
    def test_m2m_rename_field_in_target_model(self):
        class LocalTagM2MTest(Model):
            title = CharField(max_length=255)

            class Meta:
                app_label = "schema"

        class LocalM2M(Model):
            tags = ManyToManyField(LocalTagM2MTest)

            class Meta:
                app_label = "schema"

        # Create the tables.
        with connection.schema_editor() as editor:
            editor.create_model(LocalM2M)
            editor.create_model(LocalTagM2MTest)
        self.isolated_local_models = [LocalM2M, LocalTagM2MTest]
        # Ensure the m2m table is there.
        self.assertEqual(len(self.column_classes(LocalM2M)), 1)
        # Alter a field in LocalTagM2MTest.
        old_field = LocalTagM2MTest._meta.get_field("title")
        new_field = CharField(max_length=254)
        new_field.contribute_to_class(LocalTagM2MTest, "title1")
        # @isolate_apps() and inner models are needed to have the model
        # relations populated, otherwise this doesn't act as a regression test.
        self.assertEqual(len(new_field.model._meta.related_objects), 1)
        with connection.schema_editor() as editor:
            editor.alter_field(LocalTagM2MTest, old_field, new_field, strict=True)
        # Ensure the m2m table is still there.
        self.assertEqual(len(self.column_classes(LocalM2M)), 1)

    @skipUnlessDBFeature(
        "supports_column_check_constraints", "can_introspect_check_constraints"
    )
    def test_check_constraints(self):
        """
        Tests creating/deleting CHECK constraints
        """
        # Create the tables
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Ensure the constraint exists
        constraints = self.get_constraints(Author._meta.db_table)
        if not any(
            details["columns"] == ["height"] and details["check"]
            for details in constraints.values()
        ):
            self.fail("No check constraint for height found")
        # Alter the column to remove it
        old_field = Author._meta.get_field("height")
        new_field = IntegerField(null=True, blank=True)
        new_field.set_attributes_from_name("height")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        constraints = self.get_constraints(Author._meta.db_table)
        for details in constraints.values():
            if details["columns"] == ["height"] and details["check"]:
                self.fail("Check constraint for height found")
        # Alter the column to re-add it
        new_field2 = Author._meta.get_field("height")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, new_field, new_field2, strict=True)
        constraints = self.get_constraints(Author._meta.db_table)
        if not any(
            details["columns"] == ["height"] and details["check"]
            for details in constraints.values()
        ):
            self.fail("No check constraint for height found")

    @skipUnlessDBFeature(
        "supports_column_check_constraints", "can_introspect_check_constraints"
    )
    @isolate_apps("schema")
    def test_check_constraint_timedelta_param(self):
        class DurationModel(Model):
            duration = DurationField()

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(DurationModel)
        self.isolated_local_models = [DurationModel]
        constraint_name = "duration_gte_5_minutes"
        constraint = CheckConstraint(
            check=Q(duration__gt=datetime.timedelta(minutes=5)),
            name=constraint_name,
        )
        DurationModel._meta.constraints = [constraint]
        with connection.schema_editor() as editor:
            editor.add_constraint(DurationModel, constraint)
        constraints = self.get_constraints(DurationModel._meta.db_table)
        self.assertIn(constraint_name, constraints)
        with self.assertRaises(IntegrityError), atomic():
            DurationModel.objects.create(duration=datetime.timedelta(minutes=4))
        DurationModel.objects.create(duration=datetime.timedelta(minutes=10))

    @skipUnlessDBFeature(
        "supports_column_check_constraints", "can_introspect_check_constraints"
    )
    def test_remove_field_check_does_not_remove_meta_constraints(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Add the custom check constraint
        constraint = CheckConstraint(
            check=Q(height__gte=0), name="author_height_gte_0_check"
        )
        custom_constraint_name = constraint.name
        Author._meta.constraints = [constraint]
        with connection.schema_editor() as editor:
            editor.add_constraint(Author, constraint)
        # Ensure the constraints exist
        constraints = self.get_constraints(Author._meta.db_table)
        self.assertIn(custom_constraint_name, constraints)
        other_constraints = [
            name
            for name, details in constraints.items()
            if details["columns"] == ["height"]
            and details["check"]
            and name != custom_constraint_name
        ]
        self.assertEqual(len(other_constraints), 1)
        # Alter the column to remove field check
        old_field = Author._meta.get_field("height")
        new_field = IntegerField(null=True, blank=True)
        new_field.set_attributes_from_name("height")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        constraints = self.get_constraints(Author._meta.db_table)
        self.assertIn(custom_constraint_name, constraints)
        other_constraints = [
            name
            for name, details in constraints.items()
            if details["columns"] == ["height"]
            and details["check"]
            and name != custom_constraint_name
        ]
        self.assertEqual(len(other_constraints), 0)
        # Alter the column to re-add field check
        new_field2 = Author._meta.get_field("height")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, new_field, new_field2, strict=True)
        constraints = self.get_constraints(Author._meta.db_table)
        self.assertIn(custom_constraint_name, constraints)
        other_constraints = [
            name
            for name, details in constraints.items()
            if details["columns"] == ["height"]
            and details["check"]
            and name != custom_constraint_name
        ]
        self.assertEqual(len(other_constraints), 1)
        # Drop the check constraint
        with connection.schema_editor() as editor:
            Author._meta.constraints = []
            editor.remove_constraint(Author, constraint)

    def test_unique(self):
        """
        Tests removing and adding unique constraints to a single column.
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Tag)
        # Ensure the field is unique to begin with
        Tag.objects.create(title="foo", slug="foo")
        with self.assertRaises(IntegrityError):
            Tag.objects.create(title="bar", slug="foo")
        Tag.objects.all().delete()
        # Alter the slug field to be non-unique
        old_field = Tag._meta.get_field("slug")
        new_field = SlugField(unique=False)
        new_field.set_attributes_from_name("slug")
        with connection.schema_editor() as editor:
            editor.alter_field(Tag, old_field, new_field, strict=True)
        # Ensure the field is no longer unique
        Tag.objects.create(title="foo", slug="foo")
        Tag.objects.create(title="bar", slug="foo")
        Tag.objects.all().delete()
        # Alter the slug field to be unique
        new_field2 = SlugField(unique=True)
        new_field2.set_attributes_from_name("slug")
        with connection.schema_editor() as editor:
            editor.alter_field(Tag, new_field, new_field2, strict=True)
        # Ensure the field is unique again
        Tag.objects.create(title="foo", slug="foo")
        with self.assertRaises(IntegrityError):
            Tag.objects.create(title="bar", slug="foo")
        Tag.objects.all().delete()
        # Rename the field
        new_field3 = SlugField(unique=True)
        new_field3.set_attributes_from_name("slug2")
        with connection.schema_editor() as editor:
            editor.alter_field(Tag, new_field2, new_field3, strict=True)
        # Ensure the field is still unique
        TagUniqueRename.objects.create(title="foo", slug2="foo")
        with self.assertRaises(IntegrityError):
            TagUniqueRename.objects.create(title="bar", slug2="foo")
        Tag.objects.all().delete()

    def test_unique_name_quoting(self):
        old_table_name = TagUniqueRename._meta.db_table
        try:
            with connection.schema_editor() as editor:
                editor.create_model(TagUniqueRename)
                editor.alter_db_table(TagUniqueRename, old_table_name, "unique-table")
                TagUniqueRename._meta.db_table = "unique-table"
                # This fails if the unique index name isn't quoted.
                editor.alter_unique_together(TagUniqueRename, [], (("title", "slug2"),))
        finally:
            with connection.schema_editor() as editor:
                editor.delete_model(TagUniqueRename)
            TagUniqueRename._meta.db_table = old_table_name

    @isolate_apps("schema")
    @skipUnlessDBFeature("supports_foreign_keys")
    def test_unique_no_unnecessary_fk_drops(self):
        """
        If AlterField isn't selective about dropping foreign key constraints
        when modifying a field with a unique constraint, the AlterField
        incorrectly drops and recreates the Book.author foreign key even though
        it doesn't restrict the field being changed (#29193).
        """

        class Author(Model):
            name = CharField(max_length=254, unique=True)

            class Meta:
                app_label = "schema"

        class Book(Model):
            author = ForeignKey(Author, CASCADE)

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        new_field = CharField(max_length=255, unique=True)
        new_field.model = Author
        new_field.set_attributes_from_name("name")
        with self.assertLogs("django.db.backends.schema", "DEBUG") as cm:
            with connection.schema_editor() as editor:
                editor.alter_field(Author, Author._meta.get_field("name"), new_field)
        # One SQL statement is executed to alter the field.
        self.assertEqual(len(cm.records), 1)

    @isolate_apps("schema")
    def test_unique_and_reverse_m2m(self):
        """
        AlterField can modify a unique field when there's a reverse M2M
        relation on the model.
        """

        class Tag(Model):
            title = CharField(max_length=255)
            slug = SlugField(unique=True)

            class Meta:
                app_label = "schema"

        class Book(Model):
            tags = ManyToManyField(Tag, related_name="books")

            class Meta:
                app_label = "schema"

        self.isolated_local_models = [Book._meta.get_field("tags").remote_field.through]
        with connection.schema_editor() as editor:
            editor.create_model(Tag)
            editor.create_model(Book)
        new_field = SlugField(max_length=75, unique=True)
        new_field.model = Tag
        new_field.set_attributes_from_name("slug")
        with self.assertLogs("django.db.backends.schema", "DEBUG") as cm:
            with connection.schema_editor() as editor:
                editor.alter_field(Tag, Tag._meta.get_field("slug"), new_field)
        # One SQL statement is executed to alter the field.
        self.assertEqual(len(cm.records), 1)
        # Ensure that the field is still unique.
        Tag.objects.create(title="foo", slug="foo")
        with self.assertRaises(IntegrityError):
            Tag.objects.create(title="bar", slug="foo")

    def test_remove_ignored_unique_constraint_not_create_fk_index(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        constraint = UniqueConstraint(
            "author",
            condition=Q(title__in=["tHGttG", "tRatEotU"]),
            name="book_author_condition_uniq",
        )
        # Add unique constraint.
        with connection.schema_editor() as editor:
            editor.add_constraint(Book, constraint)
        old_constraints = self.get_constraints_for_column(
            Book,
            Book._meta.get_field("author").column,
        )
        # Remove unique constraint.
        with connection.schema_editor() as editor:
            editor.remove_constraint(Book, constraint)
        new_constraints = self.get_constraints_for_column(
            Book,
            Book._meta.get_field("author").column,
        )
        # Redundant foreign key index is not added.
        self.assertEqual(
            (
                len(old_constraints) - 1
                if connection.features.supports_partial_indexes
                else len(old_constraints)
            ),
            len(new_constraints),
        )

    @skipUnlessDBFeature("allows_multiple_constraints_on_same_fields")
    def test_remove_field_unique_does_not_remove_meta_constraints(self):
        with connection.schema_editor() as editor:
            editor.create_model(AuthorWithUniqueName)
        self.local_models = [AuthorWithUniqueName]
        # Add the custom unique constraint
        constraint = UniqueConstraint(fields=["name"], name="author_name_uniq")
        custom_constraint_name = constraint.name
        AuthorWithUniqueName._meta.constraints = [constraint]
        with connection.schema_editor() as editor:
            editor.add_constraint(AuthorWithUniqueName, constraint)
        # Ensure the constraints exist
        constraints = self.get_constraints(AuthorWithUniqueName._meta.db_table)
        self.assertIn(custom_constraint_name, constraints)
        other_constraints = [
            name
            for name, details in constraints.items()
            if details["columns"] == ["name"]
            and details["unique"]
            and name != custom_constraint_name
        ]
        self.assertEqual(len(other_constraints), 1)
        # Alter the column to remove field uniqueness
        old_field = AuthorWithUniqueName._meta.get_field("name")
        new_field = CharField(max_length=255)
        new_field.set_attributes_from_name("name")
        with connection.schema_editor() as editor:
            editor.alter_field(AuthorWithUniqueName, old_field, new_field, strict=True)
        constraints = self.get_constraints(AuthorWithUniqueName._meta.db_table)
        self.assertIn(custom_constraint_name, constraints)
        other_constraints = [
            name
            for name, details in constraints.items()
            if details["columns"] == ["name"]
            and details["unique"]
            and name != custom_constraint_name
        ]
        self.assertEqual(len(other_constraints), 0)
        # Alter the column to re-add field uniqueness
        new_field2 = AuthorWithUniqueName._meta.get_field("name")
        with connection.schema_editor() as editor:
            editor.alter_field(AuthorWithUniqueName, new_field, new_field2, strict=True)
        constraints = self.get_constraints(AuthorWithUniqueName._meta.db_table)
        self.assertIn(custom_constraint_name, constraints)
        other_constraints = [
            name
            for name, details in constraints.items()
            if details["columns"] == ["name"]
            and details["unique"]
            and name != custom_constraint_name
        ]
        self.assertEqual(len(other_constraints), 1)
        # Drop the unique constraint
        with connection.schema_editor() as editor:
            AuthorWithUniqueName._meta.constraints = []
            editor.remove_constraint(AuthorWithUniqueName, constraint)

    def test_unique_together(self):
        """
        Tests removing and adding unique_together constraints on a model.
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(UniqueTest)
        # Ensure the fields are unique to begin with
        UniqueTest.objects.create(year=2012, slug="foo")
        UniqueTest.objects.create(year=2011, slug="foo")
        UniqueTest.objects.create(year=2011, slug="bar")
        with self.assertRaises(IntegrityError):
            UniqueTest.objects.create(year=2012, slug="foo")
        UniqueTest.objects.all().delete()
        # Alter the model to its non-unique-together companion
        with connection.schema_editor() as editor:
            editor.alter_unique_together(
                UniqueTest, UniqueTest._meta.unique_together, []
            )
        # Ensure the fields are no longer unique
        UniqueTest.objects.create(year=2012, slug="foo")
        UniqueTest.objects.create(year=2012, slug="foo")
        UniqueTest.objects.all().delete()
        # Alter it back
        new_field2 = SlugField(unique=True)
        new_field2.set_attributes_from_name("slug")
        with connection.schema_editor() as editor:
            editor.alter_unique_together(
                UniqueTest, [], UniqueTest._meta.unique_together
            )
        # Ensure the fields are unique again
        UniqueTest.objects.create(year=2012, slug="foo")
        with self.assertRaises(IntegrityError):
            UniqueTest.objects.create(year=2012, slug="foo")
        UniqueTest.objects.all().delete()

    def test_unique_together_with_fk(self):
        """
        Tests removing and adding unique_together constraints that include
        a foreign key.
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        # Ensure the fields are unique to begin with
        self.assertEqual(Book._meta.unique_together, ())
        # Add the unique_together constraint
        with connection.schema_editor() as editor:
            editor.alter_unique_together(Book, [], [["author", "title"]])
        # Alter it back
        with connection.schema_editor() as editor:
            editor.alter_unique_together(Book, [["author", "title"]], [])

    def test_unique_together_with_fk_with_existing_index(self):
        """
        Tests removing and adding unique_together constraints that include
        a foreign key, where the foreign key is added after the model is
        created.
        """
        # Create the tables
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(BookWithoutAuthor)
            new_field = ForeignKey(Author, CASCADE)
            new_field.set_attributes_from_name("author")
            editor.add_field(BookWithoutAuthor, new_field)
        # Ensure the fields aren't unique to begin with
        self.assertEqual(Book._meta.unique_together, ())
        # Add the unique_together constraint
        with connection.schema_editor() as editor:
            editor.alter_unique_together(Book, [], [["author", "title"]])
        # Alter it back
        with connection.schema_editor() as editor:
            editor.alter_unique_together(Book, [["author", "title"]], [])

    def _test_composed_index_with_fk(self, index):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        table = Book._meta.db_table
        self.assertEqual(Book._meta.indexes, [])
        Book._meta.indexes = [index]
        with connection.schema_editor() as editor:
            editor.add_index(Book, index)
        self.assertIn(index.name, self.get_constraints(table))
        Book._meta.indexes = []
        with connection.schema_editor() as editor:
            editor.remove_index(Book, index)
        self.assertNotIn(index.name, self.get_constraints(table))

    def test_composed_index_with_fk(self):
        index = Index(fields=["author", "title"], name="book_author_title_idx")
        self._test_composed_index_with_fk(index)

    def test_composed_desc_index_with_fk(self):
        index = Index(fields=["-author", "title"], name="book_author_title_idx")
        self._test_composed_index_with_fk(index)

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_composed_func_index_with_fk(self):
        index = Index(F("author"), F("title"), name="book_author_title_idx")
        self._test_composed_index_with_fk(index)

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_composed_desc_func_index_with_fk(self):
        index = Index(F("author").desc(), F("title"), name="book_author_title_idx")
        self._test_composed_index_with_fk(index)

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_composed_func_transform_index_with_fk(self):
        index = Index(F("title__lower"), name="book_title_lower_idx")
        with register_lookup(CharField, Lower):
            self._test_composed_index_with_fk(index)

    def _test_composed_constraint_with_fk(self, constraint):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        table = Book._meta.db_table
        self.assertEqual(Book._meta.constraints, [])
        Book._meta.constraints = [constraint]
        with connection.schema_editor() as editor:
            editor.add_constraint(Book, constraint)
        self.assertIn(constraint.name, self.get_constraints(table))
        Book._meta.constraints = []
        with connection.schema_editor() as editor:
            editor.remove_constraint(Book, constraint)
        self.assertNotIn(constraint.name, self.get_constraints(table))

    def test_composed_constraint_with_fk(self):
        constraint = UniqueConstraint(
            fields=["author", "title"],
            name="book_author_title_uniq",
        )
        self._test_composed_constraint_with_fk(constraint)

    @skipUnlessDBFeature(
        "supports_column_check_constraints", "can_introspect_check_constraints"
    )
    def test_composed_check_constraint_with_fk(self):
        constraint = CheckConstraint(check=Q(author__gt=0), name="book_author_check")
        self._test_composed_constraint_with_fk(constraint)

    @skipUnlessDBFeature("allows_multiple_constraints_on_same_fields")
    def test_remove_unique_together_does_not_remove_meta_constraints(self):
        with connection.schema_editor() as editor:
            editor.create_model(AuthorWithUniqueNameAndBirthday)
        self.local_models = [AuthorWithUniqueNameAndBirthday]
        # Add the custom unique constraint
        constraint = UniqueConstraint(
            fields=["name", "birthday"], name="author_name_birthday_uniq"
        )
        custom_constraint_name = constraint.name
        AuthorWithUniqueNameAndBirthday._meta.constraints = [constraint]
        with connection.schema_editor() as editor:
            editor.add_constraint(AuthorWithUniqueNameAndBirthday, constraint)
        # Ensure the constraints exist
        constraints = self.get_constraints(
            AuthorWithUniqueNameAndBirthday._meta.db_table
        )
        self.assertIn(custom_constraint_name, constraints)
        other_constraints = [
            name
            for name, details in constraints.items()
            if details["columns"] == ["name", "birthday"]
            and details["unique"]
            and name != custom_constraint_name
        ]
        self.assertEqual(len(other_constraints), 1)
        # Remove unique together
        unique_together = AuthorWithUniqueNameAndBirthday._meta.unique_together
        with connection.schema_editor() as editor:
            editor.alter_unique_together(
                AuthorWithUniqueNameAndBirthday, unique_together, []
            )
        constraints = self.get_constraints(
            AuthorWithUniqueNameAndBirthday._meta.db_table
        )
        self.assertIn(custom_constraint_name, constraints)
        other_constraints = [
            name
            for name, details in constraints.items()
            if details["columns"] == ["name", "birthday"]
            and details["unique"]
            and name != custom_constraint_name
        ]
        self.assertEqual(len(other_constraints), 0)
        # Re-add unique together
        with connection.schema_editor() as editor:
            editor.alter_unique_together(
                AuthorWithUniqueNameAndBirthday, [], unique_together
            )
        constraints = self.get_constraints(
            AuthorWithUniqueNameAndBirthday._meta.db_table
        )
        self.assertIn(custom_constraint_name, constraints)
        other_constraints = [
            name
            for name, details in constraints.items()
            if details["columns"] == ["name", "birthday"]
            and details["unique"]
            and name != custom_constraint_name
        ]
        self.assertEqual(len(other_constraints), 1)
        # Drop the unique constraint
        with connection.schema_editor() as editor:
            AuthorWithUniqueNameAndBirthday._meta.constraints = []
            editor.remove_constraint(AuthorWithUniqueNameAndBirthday, constraint)

    def test_unique_constraint(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        constraint = UniqueConstraint(fields=["name"], name="name_uq")
        # Add constraint.
        with connection.schema_editor() as editor:
            editor.add_constraint(Author, constraint)
            sql = constraint.create_sql(Author, editor)
        table = Author._meta.db_table
        self.assertIs(sql.references_table(table), True)
        self.assertIs(sql.references_column(table, "name"), True)
        # Remove constraint.
        with connection.schema_editor() as editor:
            editor.remove_constraint(Author, constraint)
        self.assertNotIn(constraint.name, self.get_constraints(table))

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_func_unique_constraint(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        constraint = UniqueConstraint(Upper("name").desc(), name="func_upper_uq")
        # Add constraint.
        with connection.schema_editor() as editor:
            editor.add_constraint(Author, constraint)
            sql = constraint.create_sql(Author, editor)
        table = Author._meta.db_table
        constraints = self.get_constraints(table)
        if connection.features.supports_index_column_ordering:
            self.assertIndexOrder(table, constraint.name, ["DESC"])
        self.assertIn(constraint.name, constraints)
        self.assertIs(constraints[constraint.name]["unique"], True)
        # SQL contains a database function.
        self.assertIs(sql.references_column(table, "name"), True)
        self.assertIn("UPPER(%s)" % editor.quote_name("name"), str(sql))
        # Remove constraint.
        with connection.schema_editor() as editor:
            editor.remove_constraint(Author, constraint)
        self.assertNotIn(constraint.name, self.get_constraints(table))

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_composite_func_unique_constraint(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(BookWithSlug)
        constraint = UniqueConstraint(
            Upper("title"),
            Lower("slug"),
            name="func_upper_lower_unq",
        )
        # Add constraint.
        with connection.schema_editor() as editor:
            editor.add_constraint(BookWithSlug, constraint)
            sql = constraint.create_sql(BookWithSlug, editor)
        table = BookWithSlug._meta.db_table
        constraints = self.get_constraints(table)
        self.assertIn(constraint.name, constraints)
        self.assertIs(constraints[constraint.name]["unique"], True)
        # SQL contains database functions.
        self.assertIs(sql.references_column(table, "title"), True)
        self.assertIs(sql.references_column(table, "slug"), True)
        sql = str(sql)
        self.assertIn("UPPER(%s)" % editor.quote_name("title"), sql)
        self.assertIn("LOWER(%s)" % editor.quote_name("slug"), sql)
        self.assertLess(sql.index("UPPER"), sql.index("LOWER"))
        # Remove constraint.
        with connection.schema_editor() as editor:
            editor.remove_constraint(BookWithSlug, constraint)
        self.assertNotIn(constraint.name, self.get_constraints(table))

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_unique_constraint_field_and_expression(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        constraint = UniqueConstraint(
            F("height").desc(),
            "uuid",
            Lower("name").asc(),
            name="func_f_lower_field_unq",
        )
        # Add constraint.
        with connection.schema_editor() as editor:
            editor.add_constraint(Author, constraint)
            sql = constraint.create_sql(Author, editor)
        table = Author._meta.db_table
        if connection.features.supports_index_column_ordering:
            self.assertIndexOrder(table, constraint.name, ["DESC", "ASC", "ASC"])
        constraints = self.get_constraints(table)
        self.assertIs(constraints[constraint.name]["unique"], True)
        self.assertEqual(len(constraints[constraint.name]["columns"]), 3)
        self.assertEqual(constraints[constraint.name]["columns"][1], "uuid")
        # SQL contains database functions and columns.
        self.assertIs(sql.references_column(table, "height"), True)
        self.assertIs(sql.references_column(table, "name"), True)
        self.assertIs(sql.references_column(table, "uuid"), True)
        self.assertIn("LOWER(%s)" % editor.quote_name("name"), str(sql))
        # Remove constraint.
        with connection.schema_editor() as editor:
            editor.remove_constraint(Author, constraint)
        self.assertNotIn(constraint.name, self.get_constraints(table))

    @skipUnlessDBFeature("supports_expression_indexes", "supports_partial_indexes")
    def test_func_unique_constraint_partial(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        constraint = UniqueConstraint(
            Upper("name"),
            name="func_upper_cond_weight_uq",
            condition=Q(weight__isnull=False),
        )
        # Add constraint.
        with connection.schema_editor() as editor:
            editor.add_constraint(Author, constraint)
            sql = constraint.create_sql(Author, editor)
        table = Author._meta.db_table
        constraints = self.get_constraints(table)
        self.assertIn(constraint.name, constraints)
        self.assertIs(constraints[constraint.name]["unique"], True)
        self.assertIs(sql.references_column(table, "name"), True)
        self.assertIn("UPPER(%s)" % editor.quote_name("name"), str(sql))
        self.assertIn(
            "WHERE %s IS NOT NULL" % editor.quote_name("weight"),
            str(sql),
        )
        # Remove constraint.
        with connection.schema_editor() as editor:
            editor.remove_constraint(Author, constraint)
        self.assertNotIn(constraint.name, self.get_constraints(table))

    @skipUnlessDBFeature("supports_expression_indexes", "supports_covering_indexes")
    def test_func_unique_constraint_covering(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        constraint = UniqueConstraint(
            Upper("name"),
            name="func_upper_covering_uq",
            include=["weight", "height"],
        )
        # Add constraint.
        with connection.schema_editor() as editor:
            editor.add_constraint(Author, constraint)
            sql = constraint.create_sql(Author, editor)
        table = Author._meta.db_table
        constraints = self.get_constraints(table)
        self.assertIn(constraint.name, constraints)
        self.assertIs(constraints[constraint.name]["unique"], True)
        self.assertEqual(
            constraints[constraint.name]["columns"],
            [None, "weight", "height"],
        )
        self.assertIs(sql.references_column(table, "name"), True)
        self.assertIs(sql.references_column(table, "weight"), True)
        self.assertIs(sql.references_column(table, "height"), True)
        self.assertIn("UPPER(%s)" % editor.quote_name("name"), str(sql))
        self.assertIn(
            "INCLUDE (%s, %s)"
            % (
                editor.quote_name("weight"),
                editor.quote_name("height"),
            ),
            str(sql),
        )
        # Remove constraint.
        with connection.schema_editor() as editor:
            editor.remove_constraint(Author, constraint)
        self.assertNotIn(constraint.name, self.get_constraints(table))

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_func_unique_constraint_lookups(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        with register_lookup(CharField, Lower), register_lookup(IntegerField, Abs):
            constraint = UniqueConstraint(
                F("name__lower"),
                F("weight__abs"),
                name="func_lower_abs_lookup_uq",
            )
            # Add constraint.
            with connection.schema_editor() as editor:
                editor.add_constraint(Author, constraint)
                sql = constraint.create_sql(Author, editor)
            table = Author._meta.db_table
            constraints = self.get_constraints(table)
            self.assertIn(constraint.name, constraints)
            self.assertIs(constraints[constraint.name]["unique"], True)
            # SQL contains columns.
            self.assertIs(sql.references_column(table, "name"), True)
            self.assertIs(sql.references_column(table, "weight"), True)
            # Remove constraint.
            with connection.schema_editor() as editor:
                editor.remove_constraint(Author, constraint)
        self.assertNotIn(constraint.name, self.get_constraints(table))

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_func_unique_constraint_collate(self):
        collation = connection.features.test_collations.get("non_default")
        if not collation:
            self.skipTest("This backend does not support case-insensitive collations.")
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(BookWithSlug)
        constraint = UniqueConstraint(
            Collate(F("title"), collation=collation).desc(),
            Collate("slug", collation=collation),
            name="func_collate_uq",
        )
        # Add constraint.
        with connection.schema_editor() as editor:
            editor.add_constraint(BookWithSlug, constraint)
            sql = constraint.create_sql(BookWithSlug, editor)
        table = BookWithSlug._meta.db_table
        constraints = self.get_constraints(table)
        self.assertIn(constraint.name, constraints)
        self.assertIs(constraints[constraint.name]["unique"], True)
        if connection.features.supports_index_column_ordering:
            self.assertIndexOrder(table, constraint.name, ["DESC", "ASC"])
        # SQL contains columns and a collation.
        self.assertIs(sql.references_column(table, "title"), True)
        self.assertIs(sql.references_column(table, "slug"), True)
        self.assertIn("COLLATE %s" % editor.quote_name(collation), str(sql))
        # Remove constraint.
        with connection.schema_editor() as editor:
            editor.remove_constraint(BookWithSlug, constraint)
        self.assertNotIn(constraint.name, self.get_constraints(table))

    @skipIfDBFeature("supports_expression_indexes")
    def test_func_unique_constraint_unsupported(self):
        # UniqueConstraint is ignored on databases that don't support indexes on
        # expressions.
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        constraint = UniqueConstraint(F("name"), name="func_name_uq")
        with connection.schema_editor() as editor, self.assertNumQueries(0):
            self.assertIsNone(editor.add_constraint(Author, constraint))
            self.assertIsNone(editor.remove_constraint(Author, constraint))

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_func_unique_constraint_nonexistent_field(self):
        constraint = UniqueConstraint(Lower("nonexistent"), name="func_nonexistent_uq")
        msg = (
            "Cannot resolve keyword 'nonexistent' into field. Choices are: "
            "height, id, name, uuid, weight"
        )
        with self.assertRaisesMessage(FieldError, msg):
            with connection.schema_editor() as editor:
                editor.add_constraint(Author, constraint)

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_func_unique_constraint_nondeterministic(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        constraint = UniqueConstraint(Random(), name="func_random_uq")
        with connection.schema_editor() as editor:
            with self.assertRaises(DatabaseError):
                editor.add_constraint(Author, constraint)

    @skipUnlessDBFeature("supports_nulls_distinct_unique_constraints")
    def test_unique_constraint_index_nulls_distinct(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        nulls_distinct = UniqueConstraint(
            F("height"), name="distinct_height", nulls_distinct=True
        )
        nulls_not_distinct = UniqueConstraint(
            F("weight"), name="not_distinct_weight", nulls_distinct=False
        )
        with connection.schema_editor() as editor:
            editor.add_constraint(Author, nulls_distinct)
            editor.add_constraint(Author, nulls_not_distinct)
        Author.objects.create(name="", height=None, weight=None)
        Author.objects.create(name="", height=None, weight=1)
        with self.assertRaises(IntegrityError):
            Author.objects.create(name="", height=1, weight=None)
        with connection.schema_editor() as editor:
            editor.remove_constraint(Author, nulls_distinct)
            editor.remove_constraint(Author, nulls_not_distinct)
        constraints = self.get_constraints(Author._meta.db_table)
        self.assertNotIn(nulls_distinct.name, constraints)
        self.assertNotIn(nulls_not_distinct.name, constraints)

    @skipUnlessDBFeature("supports_nulls_distinct_unique_constraints")
    def test_unique_constraint_nulls_distinct(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        constraint = UniqueConstraint(
            fields=["height", "weight"], name="constraint", nulls_distinct=False
        )
        with connection.schema_editor() as editor:
            editor.add_constraint(Author, constraint)
        Author.objects.create(name="", height=None, weight=None)
        Author.objects.create(name="", height=1, weight=None)
        Author.objects.create(name="", height=None, weight=1)
        with self.assertRaises(IntegrityError):
            Author.objects.create(name="", height=None, weight=None)
        with self.assertRaises(IntegrityError):
            Author.objects.create(name="", height=1, weight=None)
        with self.assertRaises(IntegrityError):
            Author.objects.create(name="", height=None, weight=1)
        with connection.schema_editor() as editor:
            editor.remove_constraint(Author, constraint)
        constraints = self.get_constraints(Author._meta.db_table)
        self.assertNotIn(constraint.name, constraints)

    @skipUnlessDBFeature(
        "supports_nulls_distinct_unique_constraints",
        "supports_partial_indexes",
    )
    def test_unique_constraint_nulls_distinct_condition(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        constraint = UniqueConstraint(
            fields=["height", "weight"],
            name="un_height_weight_start_A",
            condition=Q(name__startswith="A"),
            nulls_distinct=False,
        )
        with connection.schema_editor() as editor:
            editor.add_constraint(Author, constraint)
        Author.objects.create(name="Adam", height=None, weight=None)
        Author.objects.create(name="Avocado", height=1, weight=None)
        Author.objects.create(name="Adrian", height=None, weight=1)
        with self.assertRaises(IntegrityError):
            Author.objects.create(name="Alex", height=None, weight=None)
        Author.objects.create(name="Bob", height=None, weight=None)
        with self.assertRaises(IntegrityError):
            Author.objects.create(name="Alex", height=1, weight=None)
        Author.objects.create(name="Bill", height=None, weight=None)
        with self.assertRaises(IntegrityError):
            Author.objects.create(name="Alex", height=None, weight=1)
        Author.objects.create(name="Celine", height=None, weight=1)
        with connection.schema_editor() as editor:
            editor.remove_constraint(Author, constraint)
        constraints = self.get_constraints(Author._meta.db_table)
        self.assertNotIn(constraint.name, constraints)

    @skipIfDBFeature("supports_nulls_distinct_unique_constraints")
    def test_unique_constraint_nulls_distinct_unsupported(self):
        # UniqueConstraint is ignored on databases that don't support
        # NULLS [NOT] DISTINCT.
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        constraint = UniqueConstraint(
            F("name"), name="func_name_uq", nulls_distinct=True
        )
        with connection.schema_editor() as editor, self.assertNumQueries(0):
            self.assertIsNone(editor.add_constraint(Author, constraint))
            self.assertIsNone(editor.remove_constraint(Author, constraint))

    @ignore_warnings(category=RemovedInDjango51Warning)
    def test_index_together(self):
        """
        Tests removing and adding index_together constraints on a model.
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Tag)
        # Ensure there's no index on the year/slug columns first
        self.assertIs(
            any(
                c["index"]
                for c in self.get_constraints("schema_tag").values()
                if c["columns"] == ["slug", "title"]
            ),
            False,
        )
        # Alter the model to add an index
        with connection.schema_editor() as editor:
            editor.alter_index_together(Tag, [], [("slug", "title")])
        # Ensure there is now an index
        self.assertIs(
            any(
                c["index"]
                for c in self.get_constraints("schema_tag").values()
                if c["columns"] == ["slug", "title"]
            ),
            True,
        )
        # Alter it back
        new_field2 = SlugField(unique=True)
        new_field2.set_attributes_from_name("slug")
        with connection.schema_editor() as editor:
            editor.alter_index_together(Tag, [("slug", "title")], [])
        # Ensure there's no index
        self.assertIs(
            any(
                c["index"]
                for c in self.get_constraints("schema_tag").values()
                if c["columns"] == ["slug", "title"]
            ),
            False,
        )

    @ignore_warnings(category=RemovedInDjango51Warning)
    def test_index_together_with_fk(self):
        """
        Tests removing and adding index_together constraints that include
        a foreign key.
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        # Ensure the fields are unique to begin with
        self.assertEqual(Book._meta.index_together, ())
        # Add the unique_together constraint
        with connection.schema_editor() as editor:
            editor.alter_index_together(Book, [], [["author", "title"]])
        # Alter it back
        with connection.schema_editor() as editor:
            editor.alter_index_together(Book, [["author", "title"]], [])

    @ignore_warnings(category=RemovedInDjango51Warning)
    @isolate_apps("schema")
    def test_create_index_together(self):
        """
        Tests creating models with index_together already defined
        """

        class TagIndexed(Model):
            title = CharField(max_length=255)
            slug = SlugField(unique=True)

            class Meta:
                app_label = "schema"
                index_together = [["slug", "title"]]

        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(TagIndexed)
        self.isolated_local_models = [TagIndexed]
        # Ensure there is an index
        self.assertIs(
            any(
                c["index"]
                for c in self.get_constraints("schema_tagindexed").values()
                if c["columns"] == ["slug", "title"]
            ),
            True,
        )

    @skipUnlessDBFeature("allows_multiple_constraints_on_same_fields")
    @ignore_warnings(category=RemovedInDjango51Warning)
    @isolate_apps("schema")
    def test_remove_index_together_does_not_remove_meta_indexes(self):
        class AuthorWithIndexedNameAndBirthday(Model):
            name = CharField(max_length=255)
            birthday = DateField()

            class Meta:
                app_label = "schema"
                index_together = [["name", "birthday"]]

        with connection.schema_editor() as editor:
            editor.create_model(AuthorWithIndexedNameAndBirthday)
        self.isolated_local_models = [AuthorWithIndexedNameAndBirthday]
        # Add the custom index
        index = Index(fields=["name", "birthday"], name="author_name_birthday_idx")
        custom_index_name = index.name
        AuthorWithIndexedNameAndBirthday._meta.indexes = [index]
        with connection.schema_editor() as editor:
            editor.add_index(AuthorWithIndexedNameAndBirthday, index)
        # Ensure the indexes exist
        constraints = self.get_constraints(
            AuthorWithIndexedNameAndBirthday._meta.db_table
        )
        self.assertIn(custom_index_name, constraints)
        other_constraints = [
            name
            for name, details in constraints.items()
            if details["columns"] == ["name", "birthday"]
            and details["index"]
            and name != custom_index_name
        ]
        self.assertEqual(len(other_constraints), 1)
        # Remove index together
        index_together = AuthorWithIndexedNameAndBirthday._meta.index_together
        with connection.schema_editor() as editor:
            editor.alter_index_together(
                AuthorWithIndexedNameAndBirthday, index_together, []
            )
        constraints = self.get_constraints(
            AuthorWithIndexedNameAndBirthday._meta.db_table
        )
        self.assertIn(custom_index_name, constraints)
        other_constraints = [
            name
            for name, details in constraints.items()
            if details["columns"] == ["name", "birthday"]
            and details["index"]
            and name != custom_index_name
        ]
        self.assertEqual(len(other_constraints), 0)
        # Re-add index together
        with connection.schema_editor() as editor:
            editor.alter_index_together(
                AuthorWithIndexedNameAndBirthday, [], index_together
            )
        constraints = self.get_constraints(
            AuthorWithIndexedNameAndBirthday._meta.db_table
        )
        self.assertIn(custom_index_name, constraints)
        other_constraints = [
            name
            for name, details in constraints.items()
            if details["columns"] == ["name", "birthday"]
            and details["index"]
            and name != custom_index_name
        ]
        self.assertEqual(len(other_constraints), 1)
        # Drop the index
        with connection.schema_editor() as editor:
            AuthorWithIndexedNameAndBirthday._meta.indexes = []
            editor.remove_index(AuthorWithIndexedNameAndBirthday, index)

    @isolate_apps("schema")
    def test_db_table(self):
        """
        Tests renaming of the table
        """

        class Author(Model):
            name = CharField(max_length=255)

            class Meta:
                app_label = "schema"

        class Book(Model):
            author = ForeignKey(Author, CASCADE)

            class Meta:
                app_label = "schema"

        # Create the table and one referring it.
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        # Ensure the table is there to begin with
        columns = self.column_classes(Author)
        self.assertEqual(
            columns["name"][0],
            connection.features.introspected_field_types["CharField"],
        )
        # Alter the table
        with connection.schema_editor() as editor:
            editor.alter_db_table(Author, "schema_author", "schema_otherauthor")
        Author._meta.db_table = "schema_otherauthor"
        columns = self.column_classes(Author)
        self.assertEqual(
            columns["name"][0],
            connection.features.introspected_field_types["CharField"],
        )
        # Ensure the foreign key reference was updated
        self.assertForeignKeyExists(Book, "author_id", "schema_otherauthor")
        # Alter the table again
        with connection.schema_editor() as editor:
            editor.alter_db_table(Author, "schema_otherauthor", "schema_author")
        # Ensure the table is still there
        Author._meta.db_table = "schema_author"
        columns = self.column_classes(Author)
        self.assertEqual(
            columns["name"][0],
            connection.features.introspected_field_types["CharField"],
        )

    def test_add_remove_index(self):
        """
        Tests index addition and removal
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Ensure the table is there and has no index
        self.assertNotIn("title", self.get_indexes(Author._meta.db_table))
        # Add the index
        index = Index(fields=["name"], name="author_title_idx")
        with connection.schema_editor() as editor:
            editor.add_index(Author, index)
        self.assertIn("name", self.get_indexes(Author._meta.db_table))
        # Drop the index
        with connection.schema_editor() as editor:
            editor.remove_index(Author, index)
        self.assertNotIn("name", self.get_indexes(Author._meta.db_table))

    def test_remove_db_index_doesnt_remove_custom_indexes(self):
        """
        Changing db_index to False doesn't remove indexes from Meta.indexes.
        """
        with connection.schema_editor() as editor:
            editor.create_model(AuthorWithIndexedName)
        self.local_models = [AuthorWithIndexedName]
        # Ensure the table has its index
        self.assertIn("name", self.get_indexes(AuthorWithIndexedName._meta.db_table))

        # Add the custom index
        index = Index(fields=["-name"], name="author_name_idx")
        author_index_name = index.name
        with connection.schema_editor() as editor:
            db_index_name = editor._create_index_name(
                table_name=AuthorWithIndexedName._meta.db_table,
                column_names=("name",),
            )
        try:
            AuthorWithIndexedName._meta.indexes = [index]
            with connection.schema_editor() as editor:
                editor.add_index(AuthorWithIndexedName, index)
            old_constraints = self.get_constraints(AuthorWithIndexedName._meta.db_table)
            self.assertIn(author_index_name, old_constraints)
            self.assertIn(db_index_name, old_constraints)
            # Change name field to db_index=False
            old_field = AuthorWithIndexedName._meta.get_field("name")
            new_field = CharField(max_length=255)
            new_field.set_attributes_from_name("name")
            with connection.schema_editor() as editor:
                editor.alter_field(
                    AuthorWithIndexedName, old_field, new_field, strict=True
                )
            new_constraints = self.get_constraints(AuthorWithIndexedName._meta.db_table)
            self.assertNotIn(db_index_name, new_constraints)
            # The index from Meta.indexes is still in the database.
            self.assertIn(author_index_name, new_constraints)
            # Drop the index
            with connection.schema_editor() as editor:
                editor.remove_index(AuthorWithIndexedName, index)
        finally:
            AuthorWithIndexedName._meta.indexes = []

    def test_order_index(self):
        """
        Indexes defined with ordering (ASC/DESC) defined on column
        """
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # The table doesn't have an index
        self.assertNotIn("title", self.get_indexes(Author._meta.db_table))
        index_name = "author_name_idx"
        # Add the index
        index = Index(fields=["name", "-weight"], name=index_name)
        with connection.schema_editor() as editor:
            editor.add_index(Author, index)
        if connection.features.supports_index_column_ordering:
            self.assertIndexOrder(Author._meta.db_table, index_name, ["ASC", "DESC"])
        # Drop the index
        with connection.schema_editor() as editor:
            editor.remove_index(Author, index)

    def test_indexes(self):
        """
        Tests creation/altering of indexes
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        # Ensure the table is there and has the right index
        self.assertIn(
            "title",
            self.get_indexes(Book._meta.db_table),
        )
        # Alter to remove the index
        old_field = Book._meta.get_field("title")
        new_field = CharField(max_length=100, db_index=False)
        new_field.set_attributes_from_name("title")
        with connection.schema_editor() as editor:
            editor.alter_field(Book, old_field, new_field, strict=True)
        # Ensure the table is there and has no index
        self.assertNotIn(
            "title",
            self.get_indexes(Book._meta.db_table),
        )
        # Alter to re-add the index
        new_field2 = Book._meta.get_field("title")
        with connection.schema_editor() as editor:
            editor.alter_field(Book, new_field, new_field2, strict=True)
        # Ensure the table is there and has the index again
        self.assertIn(
            "title",
            self.get_indexes(Book._meta.db_table),
        )
        # Add a unique column, verify that creates an implicit index
        new_field3 = BookWithSlug._meta.get_field("slug")
        with connection.schema_editor() as editor:
            editor.add_field(Book, new_field3)
        self.assertIn(
            "slug",
            self.get_uniques(Book._meta.db_table),
        )
        # Remove the unique, check the index goes with it
        new_field4 = CharField(max_length=20, unique=False)
        new_field4.set_attributes_from_name("slug")
        with connection.schema_editor() as editor:
            editor.alter_field(BookWithSlug, new_field3, new_field4, strict=True)
        self.assertNotIn(
            "slug",
            self.get_uniques(Book._meta.db_table),
        )

    def test_text_field_with_db_index(self):
        with connection.schema_editor() as editor:
            editor.create_model(AuthorTextFieldWithIndex)
        # The text_field index is present if the database supports it.
        assertion = (
            self.assertIn
            if connection.features.supports_index_on_text_field
            else self.assertNotIn
        )
        assertion(
            "text_field", self.get_indexes(AuthorTextFieldWithIndex._meta.db_table)
        )

    def _index_expressions_wrappers(self):
        index_expression = IndexExpression()
        index_expression.set_wrapper_classes(connection)
        return ", ".join(
            [
                wrapper_cls.__qualname__
                for wrapper_cls in index_expression.wrapper_classes
            ]
        )

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_func_index_multiple_wrapper_references(self):
        index = Index(OrderBy(F("name").desc(), descending=True), name="name")
        msg = (
            "Multiple references to %s can't be used in an indexed expression."
            % self._index_expressions_wrappers()
        )
        with connection.schema_editor() as editor:
            with self.assertRaisesMessage(ValueError, msg):
                editor.add_index(Author, index)

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_func_index_invalid_topmost_expressions(self):
        index = Index(Upper(F("name").desc()), name="name")
        msg = (
            "%s must be topmost expressions in an indexed expression."
            % self._index_expressions_wrappers()
        )
        with connection.schema_editor() as editor:
            with self.assertRaisesMessage(ValueError, msg):
                editor.add_index(Author, index)

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_func_index(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        index = Index(Lower("name").desc(), name="func_lower_idx")
        # Add index.
        with connection.schema_editor() as editor:
            editor.add_index(Author, index)
            sql = index.create_sql(Author, editor)
        table = Author._meta.db_table
        if connection.features.supports_index_column_ordering:
            self.assertIndexOrder(table, index.name, ["DESC"])
        # SQL contains a database function.
        self.assertIs(sql.references_column(table, "name"), True)
        self.assertIn("LOWER(%s)" % editor.quote_name("name"), str(sql))
        # Remove index.
        with connection.schema_editor() as editor:
            editor.remove_index(Author, index)
        self.assertNotIn(index.name, self.get_constraints(table))

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_func_index_f(self):
        with connection.schema_editor() as editor:
            editor.create_model(Tag)
        index = Index("slug", F("title").desc(), name="func_f_idx")
        # Add index.
        with connection.schema_editor() as editor:
            editor.add_index(Tag, index)
            sql = index.create_sql(Tag, editor)
        table = Tag._meta.db_table
        self.assertIn(index.name, self.get_constraints(table))
        if connection.features.supports_index_column_ordering:
            self.assertIndexOrder(Tag._meta.db_table, index.name, ["ASC", "DESC"])
        # SQL contains columns.
        self.assertIs(sql.references_column(table, "slug"), True)
        self.assertIs(sql.references_column(table, "title"), True)
        # Remove index.
        with connection.schema_editor() as editor:
            editor.remove_index(Tag, index)
        self.assertNotIn(index.name, self.get_constraints(table))

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_func_index_lookups(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        with register_lookup(CharField, Lower), register_lookup(IntegerField, Abs):
            index = Index(
                F("name__lower"),
                F("weight__abs"),
                name="func_lower_abs_lookup_idx",
            )
            # Add index.
            with connection.schema_editor() as editor:
                editor.add_index(Author, index)
                sql = index.create_sql(Author, editor)
        table = Author._meta.db_table
        self.assertIn(index.name, self.get_constraints(table))
        # SQL contains columns.
        self.assertIs(sql.references_column(table, "name"), True)
        self.assertIs(sql.references_column(table, "weight"), True)
        # Remove index.
        with connection.schema_editor() as editor:
            editor.remove_index(Author, index)
        self.assertNotIn(index.name, self.get_constraints(table))

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_composite_func_index(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        index = Index(Lower("name"), Upper("name"), name="func_lower_upper_idx")
        # Add index.
        with connection.schema_editor() as editor:
            editor.add_index(Author, index)
            sql = index.create_sql(Author, editor)
        table = Author._meta.db_table
        self.assertIn(index.name, self.get_constraints(table))
        # SQL contains database functions.
        self.assertIs(sql.references_column(table, "name"), True)
        sql = str(sql)
        self.assertIn("LOWER(%s)" % editor.quote_name("name"), sql)
        self.assertIn("UPPER(%s)" % editor.quote_name("name"), sql)
        self.assertLess(sql.index("LOWER"), sql.index("UPPER"))
        # Remove index.
        with connection.schema_editor() as editor:
            editor.remove_index(Author, index)
        self.assertNotIn(index.name, self.get_constraints(table))

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_composite_func_index_field_and_expression(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        index = Index(
            F("author").desc(),
            Lower("title").asc(),
            "pub_date",
            name="func_f_lower_field_idx",
        )
        # Add index.
        with connection.schema_editor() as editor:
            editor.add_index(Book, index)
            sql = index.create_sql(Book, editor)
        table = Book._meta.db_table
        constraints = self.get_constraints(table)
        if connection.features.supports_index_column_ordering:
            self.assertIndexOrder(table, index.name, ["DESC", "ASC", "ASC"])
        self.assertEqual(len(constraints[index.name]["columns"]), 3)
        self.assertEqual(constraints[index.name]["columns"][2], "pub_date")
        # SQL contains database functions and columns.
        self.assertIs(sql.references_column(table, "author_id"), True)
        self.assertIs(sql.references_column(table, "title"), True)
        self.assertIs(sql.references_column(table, "pub_date"), True)
        self.assertIn("LOWER(%s)" % editor.quote_name("title"), str(sql))
        # Remove index.
        with connection.schema_editor() as editor:
            editor.remove_index(Book, index)
        self.assertNotIn(index.name, self.get_constraints(table))

    @skipUnlessDBFeature("supports_expression_indexes")
    @isolate_apps("schema")
    def test_func_index_f_decimalfield(self):
        class Node(Model):
            value = DecimalField(max_digits=5, decimal_places=2)

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(Node)
        index = Index(F("value"), name="func_f_decimalfield_idx")
        # Add index.
        with connection.schema_editor() as editor:
            editor.add_index(Node, index)
            sql = index.create_sql(Node, editor)
        table = Node._meta.db_table
        self.assertIn(index.name, self.get_constraints(table))
        self.assertIs(sql.references_column(table, "value"), True)
        # SQL doesn't contain casting.
        self.assertNotIn("CAST", str(sql))
        # Remove index.
        with connection.schema_editor() as editor:
            editor.remove_index(Node, index)
        self.assertNotIn(index.name, self.get_constraints(table))

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_func_index_cast(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        index = Index(Cast("weight", FloatField()), name="func_cast_idx")
        # Add index.
        with connection.schema_editor() as editor:
            editor.add_index(Author, index)
            sql = index.create_sql(Author, editor)
        table = Author._meta.db_table
        self.assertIn(index.name, self.get_constraints(table))
        self.assertIs(sql.references_column(table, "weight"), True)
        # Remove index.
        with connection.schema_editor() as editor:
            editor.remove_index(Author, index)
        self.assertNotIn(index.name, self.get_constraints(table))

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_func_index_collate(self):
        collation = connection.features.test_collations.get("non_default")
        if not collation:
            self.skipTest("This backend does not support case-insensitive collations.")
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(BookWithSlug)
        index = Index(
            Collate(F("title"), collation=collation).desc(),
            Collate("slug", collation=collation),
            name="func_collate_idx",
        )
        # Add index.
        with connection.schema_editor() as editor:
            editor.add_index(BookWithSlug, index)
            sql = index.create_sql(BookWithSlug, editor)
        table = Book._meta.db_table
        self.assertIn(index.name, self.get_constraints(table))
        if connection.features.supports_index_column_ordering:
            self.assertIndexOrder(table, index.name, ["DESC", "ASC"])
        # SQL contains columns and a collation.
        self.assertIs(sql.references_column(table, "title"), True)
        self.assertIs(sql.references_column(table, "slug"), True)
        self.assertIn("COLLATE %s" % editor.quote_name(collation), str(sql))
        # Remove index.
        with connection.schema_editor() as editor:
            editor.remove_index(Book, index)
        self.assertNotIn(index.name, self.get_constraints(table))

    @skipUnlessDBFeature("supports_expression_indexes")
    @skipIfDBFeature("collate_as_index_expression")
    def test_func_index_collate_f_ordered(self):
        collation = connection.features.test_collations.get("non_default")
        if not collation:
            self.skipTest("This backend does not support case-insensitive collations.")
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        index = Index(
            Collate(F("name").desc(), collation=collation),
            name="func_collate_f_desc_idx",
        )
        # Add index.
        with connection.schema_editor() as editor:
            editor.add_index(Author, index)
            sql = index.create_sql(Author, editor)
        table = Author._meta.db_table
        self.assertIn(index.name, self.get_constraints(table))
        if connection.features.supports_index_column_ordering:
            self.assertIndexOrder(table, index.name, ["DESC"])
        # SQL contains columns and a collation.
        self.assertIs(sql.references_column(table, "name"), True)
        self.assertIn("COLLATE %s" % editor.quote_name(collation), str(sql))
        # Remove index.
        with connection.schema_editor() as editor:
            editor.remove_index(Author, index)
        self.assertNotIn(index.name, self.get_constraints(table))

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_func_index_calc(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        index = Index(F("height") / (F("weight") + Value(5)), name="func_calc_idx")
        # Add index.
        with connection.schema_editor() as editor:
            editor.add_index(Author, index)
            sql = index.create_sql(Author, editor)
        table = Author._meta.db_table
        self.assertIn(index.name, self.get_constraints(table))
        # SQL contains columns and expressions.
        self.assertIs(sql.references_column(table, "height"), True)
        self.assertIs(sql.references_column(table, "weight"), True)
        sql = str(sql)
        self.assertIs(
            sql.index(editor.quote_name("height"))
            < sql.index("/")
            < sql.index(editor.quote_name("weight"))
            < sql.index("+")
            < sql.index("5"),
            True,
        )
        # Remove index.
        with connection.schema_editor() as editor:
            editor.remove_index(Author, index)
        self.assertNotIn(index.name, self.get_constraints(table))

    @skipUnlessDBFeature("supports_expression_indexes", "supports_json_field")
    @isolate_apps("schema")
    def test_func_index_json_key_transform(self):
        class JSONModel(Model):
            field = JSONField()

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(JSONModel)
        self.isolated_local_models = [JSONModel]
        index = Index("field__some_key", name="func_json_key_idx")
        with connection.schema_editor() as editor:
            editor.add_index(JSONModel, index)
            sql = index.create_sql(JSONModel, editor)
        table = JSONModel._meta.db_table
        self.assertIn(index.name, self.get_constraints(table))
        self.assertIs(sql.references_column(table, "field"), True)
        with connection.schema_editor() as editor:
            editor.remove_index(JSONModel, index)
        self.assertNotIn(index.name, self.get_constraints(table))

    @skipUnlessDBFeature("supports_expression_indexes", "supports_json_field")
    @isolate_apps("schema")
    def test_func_index_json_key_transform_cast(self):
        class JSONModel(Model):
            field = JSONField()

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(JSONModel)
        self.isolated_local_models = [JSONModel]
        index = Index(
            Cast(KeyTextTransform("some_key", "field"), IntegerField()),
            name="func_json_key_cast_idx",
        )
        with connection.schema_editor() as editor:
            editor.add_index(JSONModel, index)
            sql = index.create_sql(JSONModel, editor)
        table = JSONModel._meta.db_table
        self.assertIn(index.name, self.get_constraints(table))
        self.assertIs(sql.references_column(table, "field"), True)
        with connection.schema_editor() as editor:
            editor.remove_index(JSONModel, index)
        self.assertNotIn(index.name, self.get_constraints(table))

    @skipIfDBFeature("supports_expression_indexes")
    def test_func_index_unsupported(self):
        # Index is ignored on databases that don't support indexes on
        # expressions.
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        index = Index(F("name"), name="random_idx")
        with connection.schema_editor() as editor, self.assertNumQueries(0):
            self.assertIsNone(editor.add_index(Author, index))
            self.assertIsNone(editor.remove_index(Author, index))

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_func_index_nonexistent_field(self):
        index = Index(Lower("nonexistent"), name="func_nonexistent_idx")
        msg = (
            "Cannot resolve keyword 'nonexistent' into field. Choices are: "
            "height, id, name, uuid, weight"
        )
        with self.assertRaisesMessage(FieldError, msg):
            with connection.schema_editor() as editor:
                editor.add_index(Author, index)

    @skipUnlessDBFeature("supports_expression_indexes")
    def test_func_index_nondeterministic(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        index = Index(Random(), name="func_random_idx")
        with connection.schema_editor() as editor:
            with self.assertRaises(DatabaseError):
                editor.add_index(Author, index)

    def test_primary_key(self):
        """
        Tests altering of the primary key
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Tag)
        # Ensure the table is there and has the right PK
        self.assertEqual(self.get_primary_key(Tag._meta.db_table), "id")
        # Alter to change the PK
        id_field = Tag._meta.get_field("id")
        old_field = Tag._meta.get_field("slug")
        new_field = SlugField(primary_key=True)
        new_field.set_attributes_from_name("slug")
        new_field.model = Tag
        with connection.schema_editor() as editor:
            editor.remove_field(Tag, id_field)
            editor.alter_field(Tag, old_field, new_field)
        # Ensure the PK changed
        self.assertNotIn(
            "id",
            self.get_indexes(Tag._meta.db_table),
        )
        self.assertEqual(self.get_primary_key(Tag._meta.db_table), "slug")

    def test_alter_primary_key_the_same_name(self):
        with connection.schema_editor() as editor:
            editor.create_model(Thing)

        old_field = Thing._meta.get_field("when")
        new_field = CharField(max_length=2, primary_key=True)
        new_field.set_attributes_from_name("when")
        new_field.model = Thing
        with connection.schema_editor() as editor:
            editor.alter_field(Thing, old_field, new_field, strict=True)
        self.assertEqual(self.get_primary_key(Thing._meta.db_table), "when")
        with connection.schema_editor() as editor:
            editor.alter_field(Thing, new_field, old_field, strict=True)
        self.assertEqual(self.get_primary_key(Thing._meta.db_table), "when")

    def test_context_manager_exit(self):
        """
        Ensures transaction is correctly closed when an error occurs
        inside a SchemaEditor context.
        """

        class SomeError(Exception):
            pass

        try:
            with connection.schema_editor():
                raise SomeError
        except SomeError:
            self.assertFalse(connection.in_atomic_block)

    @skipIfDBFeature("can_rollback_ddl")
    def test_unsupported_transactional_ddl_disallowed(self):
        message = (
            "Executing DDL statements while in a transaction on databases "
            "that can't perform a rollback is prohibited."
        )
        with atomic(), connection.schema_editor() as editor:
            with self.assertRaisesMessage(TransactionManagementError, message):
                editor.execute(
                    editor.sql_create_table % {"table": "foo", "definition": ""}
                )

    @skipUnlessDBFeature("supports_foreign_keys", "indexes_foreign_keys")
    def test_foreign_key_index_long_names_regression(self):
        """
        Regression test for #21497.
        Only affects databases that supports foreign keys.
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(AuthorWithEvenLongerName)
            editor.create_model(BookWithLongName)
        # Find the properly shortened column name
        column_name = connection.ops.quote_name(
            "author_foreign_key_with_really_long_field_name_id"
        )
        column_name = column_name[1:-1].lower()  # unquote, and, for Oracle, un-upcase
        # Ensure the table is there and has an index on the column
        self.assertIn(
            column_name,
            self.get_indexes(BookWithLongName._meta.db_table),
        )

    @skipUnlessDBFeature("supports_foreign_keys")
    def test_add_foreign_key_long_names(self):
        """
        Regression test for #23009.
        Only affects databases that supports foreign keys.
        """
        # Create the initial tables
        with connection.schema_editor() as editor:
            editor.create_model(AuthorWithEvenLongerName)
            editor.create_model(BookWithLongName)
        # Add a second FK, this would fail due to long ref name before the fix
        new_field = ForeignKey(
            AuthorWithEvenLongerName, CASCADE, related_name="something"
        )
        new_field.set_attributes_from_name(
            "author_other_really_long_named_i_mean_so_long_fk"
        )
        with connection.schema_editor() as editor:
            editor.add_field(BookWithLongName, new_field)

    @isolate_apps("schema")
    @skipUnlessDBFeature("supports_foreign_keys")
    def test_add_foreign_key_quoted_db_table(self):
        class Author(Model):
            class Meta:
                db_table = '"table_author_double_quoted"'
                app_label = "schema"

        class Book(Model):
            author = ForeignKey(Author, CASCADE)

            class Meta:
                app_label = "schema"

        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        self.isolated_local_models = [Author]
        if connection.vendor == "mysql":
            self.assertForeignKeyExists(
                Book, "author_id", '"table_author_double_quoted"'
            )
        else:
            self.assertForeignKeyExists(Book, "author_id", "table_author_double_quoted")

    def test_add_foreign_object(self):
        with connection.schema_editor() as editor:
            editor.create_model(BookForeignObj)
        self.local_models = [BookForeignObj]

        new_field = ForeignObject(
            Author, on_delete=CASCADE, from_fields=["author_id"], to_fields=["id"]
        )
        new_field.set_attributes_from_name("author")
        with connection.schema_editor() as editor:
            editor.add_field(BookForeignObj, new_field)

    def test_creation_deletion_reserved_names(self):
        """
        Tries creating a model's table, and then deleting it when it has a
        SQL reserved name.
        """
        # Create the table
        with connection.schema_editor() as editor:
            try:
                editor.create_model(Thing)
            except OperationalError as e:
                self.fail(
                    "Errors when applying initial migration for a model "
                    "with a table named after an SQL reserved word: %s" % e
                )
        # The table is there
        list(Thing.objects.all())
        # Clean up that table
        with connection.schema_editor() as editor:
            editor.delete_model(Thing)
        # The table is gone
        with self.assertRaises(DatabaseError):
            list(Thing.objects.all())

    def test_remove_constraints_capital_letters(self):
        """
        #23065 - Constraint names must be quoted if they contain capital letters.
        """

        def get_field(*args, field_class=IntegerField, **kwargs):
            kwargs["db_column"] = "CamelCase"
            field = field_class(*args, **kwargs)
            field.set_attributes_from_name("CamelCase")
            return field

        model = Author
        field = get_field()
        table = model._meta.db_table
        column = field.column
        identifier_converter = connection.introspection.identifier_converter

        with connection.schema_editor() as editor:
            editor.create_model(model)
            editor.add_field(model, field)

            constraint_name = "CamelCaseIndex"
            expected_constraint_name = identifier_converter(constraint_name)
            editor.execute(
                editor.sql_create_index
                % {
                    "table": editor.quote_name(table),
                    "name": editor.quote_name(constraint_name),
                    "using": "",
                    "columns": editor.quote_name(column),
                    "extra": "",
                    "condition": "",
                    "include": "",
                }
            )
            self.assertIn(
                expected_constraint_name, self.get_constraints(model._meta.db_table)
            )
            editor.alter_field(model, get_field(db_index=True), field, strict=True)
            self.assertNotIn(
                expected_constraint_name, self.get_constraints(model._meta.db_table)
            )

            constraint_name = "CamelCaseUniqConstraint"
            expected_constraint_name = identifier_converter(constraint_name)
            editor.execute(editor._create_unique_sql(model, [field], constraint_name))
            self.assertIn(
                expected_constraint_name, self.get_constraints(model._meta.db_table)
            )
            editor.alter_field(model, get_field(unique=True), field, strict=True)
            self.assertNotIn(
                expected_constraint_name, self.get_constraints(model._meta.db_table)
            )

            if editor.sql_create_fk and connection.features.can_introspect_foreign_keys:
                constraint_name = "CamelCaseFKConstraint"
                expected_constraint_name = identifier_converter(constraint_name)
                editor.execute(
                    editor.sql_create_fk
                    % {
                        "table": editor.quote_name(table),
                        "name": editor.quote_name(constraint_name),
                        "column": editor.quote_name(column),
                        "to_table": editor.quote_name(table),
                        "to_column": editor.quote_name(model._meta.auto_field.column),
                        "deferrable": connection.ops.deferrable_sql(),
                    }
                )
                self.assertIn(
                    expected_constraint_name, self.get_constraints(model._meta.db_table)
                )
                editor.alter_field(
                    model,
                    get_field(Author, CASCADE, field_class=ForeignKey),
                    field,
                    strict=True,
                )
                self.assertNotIn(
                    expected_constraint_name, self.get_constraints(model._meta.db_table)
                )

    def test_add_field_use_effective_default(self):
        """
        #23987 - effective_default() should be used as the field default when
        adding a new field.
        """
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Ensure there's no surname field
        columns = self.column_classes(Author)
        self.assertNotIn("surname", columns)
        # Create a row
        Author.objects.create(name="Anonymous1")
        # Add new CharField to ensure default will be used from effective_default
        new_field = CharField(max_length=15, blank=True)
        new_field.set_attributes_from_name("surname")
        with connection.schema_editor() as editor:
            editor.add_field(Author, new_field)
        # Ensure field was added with the right default
        with connection.cursor() as cursor:
            cursor.execute("SELECT surname FROM schema_author;")
            item = cursor.fetchall()[0]
            self.assertEqual(
                item[0],
                None if connection.features.interprets_empty_strings_as_nulls else "",
            )

    def test_add_field_default_dropped(self):
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Ensure there's no surname field
        columns = self.column_classes(Author)
        self.assertNotIn("surname", columns)
        # Create a row
        Author.objects.create(name="Anonymous1")
        # Add new CharField with a default
        new_field = CharField(max_length=15, blank=True, default="surname default")
        new_field.set_attributes_from_name("surname")
        with connection.schema_editor() as editor:
            editor.add_field(Author, new_field)
        # Ensure field was added with the right default
        with connection.cursor() as cursor:
            cursor.execute("SELECT surname FROM schema_author;")
            item = cursor.fetchall()[0]
            self.assertEqual(item[0], "surname default")
            # And that the default is no longer set in the database.
            field = next(
                f
                for f in connection.introspection.get_table_description(
                    cursor, "schema_author"
                )
                if f.name == "surname"
            )
            if connection.features.can_introspect_default:
                self.assertIsNone(field.default)

    def test_add_field_default_nullable(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Add new nullable CharField with a default.
        new_field = CharField(max_length=15, blank=True, null=True, default="surname")
        new_field.set_attributes_from_name("surname")
        with connection.schema_editor() as editor:
            editor.add_field(Author, new_field)
        Author.objects.create(name="Anonymous1")
        with connection.cursor() as cursor:
            cursor.execute("SELECT surname FROM schema_author;")
            item = cursor.fetchall()[0]
            self.assertIsNone(item[0])
            field = next(
                f
                for f in connection.introspection.get_table_description(
                    cursor,
                    "schema_author",
                )
                if f.name == "surname"
            )
            # Field is still nullable.
            self.assertTrue(field.null_ok)
            # The database default is no longer set.
            if connection.features.can_introspect_default:
                self.assertIn(field.default, ["NULL", None])

    def test_add_textfield_default_nullable(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Add new nullable TextField with a default.
        new_field = TextField(blank=True, null=True, default="text")
        new_field.set_attributes_from_name("description")
        with connection.schema_editor() as editor:
            editor.add_field(Author, new_field)
        Author.objects.create(name="Anonymous1")
        with connection.cursor() as cursor:
            cursor.execute("SELECT description FROM schema_author;")
            item = cursor.fetchall()[0]
            self.assertIsNone(item[0])
            field = next(
                f
                for f in connection.introspection.get_table_description(
                    cursor,
                    "schema_author",
                )
                if f.name == "description"
            )
            # Field is still nullable.
            self.assertTrue(field.null_ok)
            # The database default is no longer set.
            if connection.features.can_introspect_default:
                self.assertIn(field.default, ["NULL", None])

    def test_alter_field_default_dropped(self):
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Create a row
        Author.objects.create(name="Anonymous1")
        self.assertIsNone(Author.objects.get().height)
        old_field = Author._meta.get_field("height")
        # The default from the new field is used in updating existing rows.
        new_field = IntegerField(blank=True, default=42)
        new_field.set_attributes_from_name("height")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        self.assertEqual(Author.objects.get().height, 42)
        # The database default should be removed.
        with connection.cursor() as cursor:
            field = next(
                f
                for f in connection.introspection.get_table_description(
                    cursor, "schema_author"
                )
                if f.name == "height"
            )
            if connection.features.can_introspect_default:
                self.assertIsNone(field.default)

    def test_alter_field_default_doesnt_perform_queries(self):
        """
        No queries are performed if a field default changes and the field's
        not changing from null to non-null.
        """
        with connection.schema_editor() as editor:
            editor.create_model(AuthorWithDefaultHeight)
        old_field = AuthorWithDefaultHeight._meta.get_field("height")
        new_default = old_field.default * 2
        new_field = PositiveIntegerField(null=True, blank=True, default=new_default)
        new_field.set_attributes_from_name("height")
        with connection.schema_editor() as editor, self.assertNumQueries(0):
            editor.alter_field(
                AuthorWithDefaultHeight, old_field, new_field, strict=True
            )

    @skipUnlessDBFeature("supports_foreign_keys")
    def test_alter_field_fk_attributes_noop(self):
        """
        No queries are performed when changing field attributes that don't
        affect the schema.
        """
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
        old_field = Book._meta.get_field("author")
        new_field = ForeignKey(
            Author,
            blank=True,
            editable=False,
            error_messages={"invalid": "error message"},
            help_text="help text",
            limit_choices_to={"limit": "choice"},
            on_delete=PROTECT,
            related_name="related_name",
            related_query_name="related_query_name",
            validators=[lambda x: x],
            verbose_name="verbose name",
        )
        new_field.set_attributes_from_name("author")
        with connection.schema_editor() as editor, self.assertNumQueries(0):
            editor.alter_field(Book, old_field, new_field, strict=True)
        with connection.schema_editor() as editor, self.assertNumQueries(0):
            editor.alter_field(Book, new_field, old_field, strict=True)

    def test_alter_field_choices_noop(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        old_field = Author._meta.get_field("name")
        new_field = CharField(
            choices=(("Jane", "Jane"), ("Joe", "Joe")),
            max_length=255,
        )
        new_field.set_attributes_from_name("name")
        with connection.schema_editor() as editor, self.assertNumQueries(0):
            editor.alter_field(Author, old_field, new_field, strict=True)
        with connection.schema_editor() as editor, self.assertNumQueries(0):
            editor.alter_field(Author, new_field, old_field, strict=True)

    def test_add_textfield_unhashable_default(self):
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Create a row
        Author.objects.create(name="Anonymous1")
        # Create a field that has an unhashable default
        new_field = TextField(default={})
        new_field.set_attributes_from_name("info")
        with connection.schema_editor() as editor:
            editor.add_field(Author, new_field)

    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    def test_add_indexed_charfield(self):
        field = CharField(max_length=255, db_index=True)
        field.set_attributes_from_name("nom_de_plume")
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.add_field(Author, field)
        # Should create two indexes; one for like operator.
        self.assertEqual(
            self.get_constraints_for_column(Author, "nom_de_plume"),
            [
                "schema_author_nom_de_plume_7570a851",
                "schema_author_nom_de_plume_7570a851_like",
            ],
        )

    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    def test_add_unique_charfield(self):
        field = CharField(max_length=255, unique=True)
        field.set_attributes_from_name("nom_de_plume")
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.add_field(Author, field)
        # Should create two indexes; one for like operator.
        self.assertEqual(
            self.get_constraints_for_column(Author, "nom_de_plume"),
            [
                "schema_author_nom_de_plume_7570a851_like",
                "schema_author_nom_de_plume_key",
            ],
        )

    @skipUnlessDBFeature("supports_comments")
    def test_add_db_comment_charfield(self):
        comment = "Custom comment"
        field = CharField(max_length=255, db_comment=comment)
        field.set_attributes_from_name("name_with_comment")
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.add_field(Author, field)
        self.assertEqual(
            self.get_column_comment(Author._meta.db_table, "name_with_comment"),
            comment,
        )

    @skipUnlessDBFeature("supports_comments")
    def test_add_db_comment_and_default_charfield(self):
        comment = "Custom comment with default"
        field = CharField(max_length=255, default="Joe Doe", db_comment=comment)
        field.set_attributes_from_name("name_with_comment_default")
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            Author.objects.create(name="Before adding a new field")
            editor.add_field(Author, field)

        self.assertEqual(
            self.get_column_comment(Author._meta.db_table, "name_with_comment_default"),
            comment,
        )
        with connection.cursor() as cursor:
            cursor.execute(
                f"SELECT name_with_comment_default FROM {Author._meta.db_table};"
            )
            for row in cursor.fetchall():
                self.assertEqual(row[0], "Joe Doe")

    @skipUnlessDBFeature("supports_comments")
    def test_alter_db_comment(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Add comment.
        old_field = Author._meta.get_field("name")
        new_field = CharField(max_length=255, db_comment="Custom comment")
        new_field.set_attributes_from_name("name")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        self.assertEqual(
            self.get_column_comment(Author._meta.db_table, "name"),
            "Custom comment",
        )
        # Alter comment.
        old_field = new_field
        new_field = CharField(max_length=255, db_comment="New custom comment")
        new_field.set_attributes_from_name("name")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        self.assertEqual(
            self.get_column_comment(Author._meta.db_table, "name"),
            "New custom comment",
        )
        # Remove comment.
        old_field = new_field
        new_field = CharField(max_length=255)
        new_field.set_attributes_from_name("name")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        self.assertIn(
            self.get_column_comment(Author._meta.db_table, "name"),
            [None, ""],
        )

    @skipUnlessDBFeature("supports_comments", "supports_foreign_keys")
    def test_alter_db_comment_foreign_key(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)

        comment = "FK custom comment"
        old_field = Book._meta.get_field("author")
        new_field = ForeignKey(Author, CASCADE, db_comment=comment)
        new_field.set_attributes_from_name("author")
        with connection.schema_editor() as editor:
            editor.alter_field(Book, old_field, new_field, strict=True)
        self.assertEqual(
            self.get_column_comment(Book._meta.db_table, "author_id"),
            comment,
        )

    @skipUnlessDBFeature("supports_comments")
    def test_alter_field_type_preserve_comment(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)

        comment = "This is the name."
        old_field = Author._meta.get_field("name")
        new_field = CharField(max_length=255, db_comment=comment)
        new_field.set_attributes_from_name("name")
        new_field.model = Author
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        self.assertEqual(
            self.get_column_comment(Author._meta.db_table, "name"),
            comment,
        )
        # Changing a field type should preserve the comment.
        old_field = new_field
        new_field = CharField(max_length=511, db_comment=comment)
        new_field.set_attributes_from_name("name")
        new_field.model = Author
        with connection.schema_editor() as editor:
            editor.alter_field(Author, new_field, old_field, strict=True)
        # Comment is preserved.
        self.assertEqual(
            self.get_column_comment(Author._meta.db_table, "name"),
            comment,
        )

    @isolate_apps("schema")
    @skipUnlessDBFeature("supports_comments")
    def test_db_comment_table(self):
        class ModelWithDbTableComment(Model):
            class Meta:
                app_label = "schema"
                db_table_comment = "Custom table comment"

        with connection.schema_editor() as editor:
            editor.create_model(ModelWithDbTableComment)
        self.isolated_local_models = [ModelWithDbTableComment]
        self.assertEqual(
            self.get_table_comment(ModelWithDbTableComment._meta.db_table),
            "Custom table comment",
        )
        # Alter table comment.
        old_db_table_comment = ModelWithDbTableComment._meta.db_table_comment
        with connection.schema_editor() as editor:
            editor.alter_db_table_comment(
                ModelWithDbTableComment, old_db_table_comment, "New table comment"
            )
        self.assertEqual(
            self.get_table_comment(ModelWithDbTableComment._meta.db_table),
            "New table comment",
        )
        # Remove table comment.
        old_db_table_comment = ModelWithDbTableComment._meta.db_table_comment
        with connection.schema_editor() as editor:
            editor.alter_db_table_comment(
                ModelWithDbTableComment, old_db_table_comment, None
            )
        self.assertIn(
            self.get_table_comment(ModelWithDbTableComment._meta.db_table),
            [None, ""],
        )

    @isolate_apps("schema")
    @skipIfDBFeature("supports_comments")
    def test_db_comment_table_unsupported(self):
        class ModelWithDbTableComment(Model):
            class Meta:
                app_label = "schema"
                db_table_comment = "Custom table comment"

        # Table comments are ignored on databases that don't support them.
        with connection.schema_editor() as editor, self.assertNumQueries(1):
            editor.create_model(ModelWithDbTableComment)
        self.isolated_local_models = [ModelWithDbTableComment]
        with connection.schema_editor() as editor, self.assertNumQueries(0):
            editor.alter_db_table_comment(
                ModelWithDbTableComment, "Custom table comment", "New table comment"
            )

    @isolate_apps("schema")
    @skipUnlessDBFeature("supports_comments", "supports_foreign_keys")
    def test_db_comments_from_abstract_model(self):
        class AbstractModelWithDbComments(Model):
            name = CharField(
                max_length=255, db_comment="Custom comment", null=True, blank=True
            )

            class Meta:
                app_label = "schema"
                abstract = True
                db_table_comment = "Custom table comment"

        class ModelWithDbComments(AbstractModelWithDbComments):
            pass

        with connection.schema_editor() as editor:
            editor.create_model(ModelWithDbComments)
        self.isolated_local_models = [ModelWithDbComments]

        self.assertEqual(
            self.get_column_comment(ModelWithDbComments._meta.db_table, "name"),
            "Custom comment",
        )
        self.assertEqual(
            self.get_table_comment(ModelWithDbComments._meta.db_table),
            "Custom table comment",
        )

    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    def test_alter_field_add_index_to_charfield(self):
        # Create the table and verify no initial indexes.
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        self.assertEqual(self.get_constraints_for_column(Author, "name"), [])
        # Alter to add db_index=True and create 2 indexes.
        old_field = Author._meta.get_field("name")
        new_field = CharField(max_length=255, db_index=True)
        new_field.set_attributes_from_name("name")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        self.assertEqual(
            self.get_constraints_for_column(Author, "name"),
            ["schema_author_name_1fbc5617", "schema_author_name_1fbc5617_like"],
        )
        # Remove db_index=True to drop both indexes.
        with connection.schema_editor() as editor:
            editor.alter_field(Author, new_field, old_field, strict=True)
        self.assertEqual(self.get_constraints_for_column(Author, "name"), [])

    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    def test_alter_field_add_unique_to_charfield(self):
        # Create the table and verify no initial indexes.
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        self.assertEqual(self.get_constraints_for_column(Author, "name"), [])
        # Alter to add unique=True and create 2 indexes.
        old_field = Author._meta.get_field("name")
        new_field = CharField(max_length=255, unique=True)
        new_field.set_attributes_from_name("name")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        self.assertEqual(
            self.get_constraints_for_column(Author, "name"),
            ["schema_author_name_1fbc5617_like", "schema_author_name_1fbc5617_uniq"],
        )
        # Remove unique=True to drop both indexes.
        with connection.schema_editor() as editor:
            editor.alter_field(Author, new_field, old_field, strict=True)
        self.assertEqual(self.get_constraints_for_column(Author, "name"), [])

    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    def test_alter_field_add_index_to_textfield(self):
        # Create the table and verify no initial indexes.
        with connection.schema_editor() as editor:
            editor.create_model(Note)
        self.assertEqual(self.get_constraints_for_column(Note, "info"), [])
        # Alter to add db_index=True and create 2 indexes.
        old_field = Note._meta.get_field("info")
        new_field = TextField(db_index=True)
        new_field.set_attributes_from_name("info")
        with connection.schema_editor() as editor:
            editor.alter_field(Note, old_field, new_field, strict=True)
        self.assertEqual(
            self.get_constraints_for_column(Note, "info"),
            ["schema_note_info_4b0ea695", "schema_note_info_4b0ea695_like"],
        )
        # Remove db_index=True to drop both indexes.
        with connection.schema_editor() as editor:
            editor.alter_field(Note, new_field, old_field, strict=True)
        self.assertEqual(self.get_constraints_for_column(Note, "info"), [])

    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    def test_alter_field_add_unique_to_charfield_with_db_index(self):
        # Create the table and verify initial indexes.
        with connection.schema_editor() as editor:
            editor.create_model(BookWithoutAuthor)
        self.assertEqual(
            self.get_constraints_for_column(BookWithoutAuthor, "title"),
            ["schema_book_title_2dfb2dff", "schema_book_title_2dfb2dff_like"],
        )
        # Alter to add unique=True (should replace the index)
        old_field = BookWithoutAuthor._meta.get_field("title")
        new_field = CharField(max_length=100, db_index=True, unique=True)
        new_field.set_attributes_from_name("title")
        with connection.schema_editor() as editor:
            editor.alter_field(BookWithoutAuthor, old_field, new_field, strict=True)
        self.assertEqual(
            self.get_constraints_for_column(BookWithoutAuthor, "title"),
            ["schema_book_title_2dfb2dff_like", "schema_book_title_2dfb2dff_uniq"],
        )
        # Alter to remove unique=True (should drop unique index)
        new_field2 = CharField(max_length=100, db_index=True)
        new_field2.set_attributes_from_name("title")
        with connection.schema_editor() as editor:
            editor.alter_field(BookWithoutAuthor, new_field, new_field2, strict=True)
        self.assertEqual(
            self.get_constraints_for_column(BookWithoutAuthor, "title"),
            ["schema_book_title_2dfb2dff", "schema_book_title_2dfb2dff_like"],
        )

    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    def test_alter_field_remove_unique_and_db_index_from_charfield(self):
        # Create the table and verify initial indexes.
        with connection.schema_editor() as editor:
            editor.create_model(BookWithoutAuthor)
        self.assertEqual(
            self.get_constraints_for_column(BookWithoutAuthor, "title"),
            ["schema_book_title_2dfb2dff", "schema_book_title_2dfb2dff_like"],
        )
        # Alter to add unique=True (should replace the index)
        old_field = BookWithoutAuthor._meta.get_field("title")
        new_field = CharField(max_length=100, db_index=True, unique=True)
        new_field.set_attributes_from_name("title")
        with connection.schema_editor() as editor:
            editor.alter_field(BookWithoutAuthor, old_field, new_field, strict=True)
        self.assertEqual(
            self.get_constraints_for_column(BookWithoutAuthor, "title"),
            ["schema_book_title_2dfb2dff_like", "schema_book_title_2dfb2dff_uniq"],
        )
        # Alter to remove both unique=True and db_index=True (should drop all indexes)
        new_field2 = CharField(max_length=100)
        new_field2.set_attributes_from_name("title")
        with connection.schema_editor() as editor:
            editor.alter_field(BookWithoutAuthor, new_field, new_field2, strict=True)
        self.assertEqual(
            self.get_constraints_for_column(BookWithoutAuthor, "title"), []
        )

    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    def test_alter_field_swap_unique_and_db_index_with_charfield(self):
        # Create the table and verify initial indexes.
        with connection.schema_editor() as editor:
            editor.create_model(BookWithoutAuthor)
        self.assertEqual(
            self.get_constraints_for_column(BookWithoutAuthor, "title"),
            ["schema_book_title_2dfb2dff", "schema_book_title_2dfb2dff_like"],
        )
        # Alter to set unique=True and remove db_index=True (should replace the index)
        old_field = BookWithoutAuthor._meta.get_field("title")
        new_field = CharField(max_length=100, unique=True)
        new_field.set_attributes_from_name("title")
        with connection.schema_editor() as editor:
            editor.alter_field(BookWithoutAuthor, old_field, new_field, strict=True)
        self.assertEqual(
            self.get_constraints_for_column(BookWithoutAuthor, "title"),
            ["schema_book_title_2dfb2dff_like", "schema_book_title_2dfb2dff_uniq"],
        )
        # Alter to set db_index=True and remove unique=True (should restore index)
        new_field2 = CharField(max_length=100, db_index=True)
        new_field2.set_attributes_from_name("title")
        with connection.schema_editor() as editor:
            editor.alter_field(BookWithoutAuthor, new_field, new_field2, strict=True)
        self.assertEqual(
            self.get_constraints_for_column(BookWithoutAuthor, "title"),
            ["schema_book_title_2dfb2dff", "schema_book_title_2dfb2dff_like"],
        )

    @unittest.skipUnless(connection.vendor == "postgresql", "PostgreSQL specific")
    def test_alter_field_add_db_index_to_charfield_with_unique(self):
        # Create the table and verify initial indexes.
        with connection.schema_editor() as editor:
            editor.create_model(Tag)
        self.assertEqual(
            self.get_constraints_for_column(Tag, "slug"),
            ["schema_tag_slug_2c418ba3_like", "schema_tag_slug_key"],
        )
        # Alter to add db_index=True
        old_field = Tag._meta.get_field("slug")
        new_field = SlugField(db_index=True, unique=True)
        new_field.set_attributes_from_name("slug")
        with connection.schema_editor() as editor:
            editor.alter_field(Tag, old_field, new_field, strict=True)
        self.assertEqual(
            self.get_constraints_for_column(Tag, "slug"),
            ["schema_tag_slug_2c418ba3_like", "schema_tag_slug_key"],
        )
        # Alter to remove db_index=True
        new_field2 = SlugField(unique=True)
        new_field2.set_attributes_from_name("slug")
        with connection.schema_editor() as editor:
            editor.alter_field(Tag, new_field, new_field2, strict=True)
        self.assertEqual(
            self.get_constraints_for_column(Tag, "slug"),
            ["schema_tag_slug_2c418ba3_like", "schema_tag_slug_key"],
        )

    def test_alter_field_add_index_to_integerfield(self):
        # Create the table and verify no initial indexes.
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        self.assertEqual(self.get_constraints_for_column(Author, "weight"), [])

        # Alter to add db_index=True and create index.
        old_field = Author._meta.get_field("weight")
        new_field = IntegerField(null=True, db_index=True)
        new_field.set_attributes_from_name("weight")
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        self.assertEqual(
            self.get_constraints_for_column(Author, "weight"),
            ["schema_author_weight_587740f9"],
        )

        # Remove db_index=True to drop index.
        with connection.schema_editor() as editor:
            editor.alter_field(Author, new_field, old_field, strict=True)
        self.assertEqual(self.get_constraints_for_column(Author, "weight"), [])

    def test_alter_pk_with_self_referential_field(self):
        """
        Changing the primary key field name of a model with a self-referential
        foreign key (#26384).
        """
        with connection.schema_editor() as editor:
            editor.create_model(Node)
        old_field = Node._meta.get_field("node_id")
        new_field = AutoField(primary_key=True)
        new_field.set_attributes_from_name("id")
        with connection.schema_editor() as editor:
            editor.alter_field(Node, old_field, new_field, strict=True)
        self.assertForeignKeyExists(Node, "parent_id", Node._meta.db_table)

    @mock.patch("django.db.backends.base.schema.datetime")
    @mock.patch("django.db.backends.base.schema.timezone")
    def test_add_datefield_and_datetimefield_use_effective_default(
        self, mocked_datetime, mocked_tz
    ):
        """
        effective_default() should be used for DateField, DateTimeField, and
        TimeField if auto_now or auto_now_add is set (#25005).
        """
        now = datetime.datetime(month=1, day=1, year=2000, hour=1, minute=1)
        now_tz = datetime.datetime(
            month=1, day=1, year=2000, hour=1, minute=1, tzinfo=datetime.timezone.utc
        )
        mocked_datetime.now = mock.MagicMock(return_value=now)
        mocked_tz.now = mock.MagicMock(return_value=now_tz)
        # Create the table
        with connection.schema_editor() as editor:
            editor.create_model(Author)
        # Check auto_now/auto_now_add attributes are not defined
        columns = self.column_classes(Author)
        self.assertNotIn("dob_auto_now", columns)
        self.assertNotIn("dob_auto_now_add", columns)
        self.assertNotIn("dtob_auto_now", columns)
        self.assertNotIn("dtob_auto_now_add", columns)
        self.assertNotIn("tob_auto_now", columns)
        self.assertNotIn("tob_auto_now_add", columns)
        # Create a row
        Author.objects.create(name="Anonymous1")
        # Ensure fields were added with the correct defaults
        dob_auto_now = DateField(auto_now=True)
        dob_auto_now.set_attributes_from_name("dob_auto_now")
        self.check_added_field_default(
            editor,
            Author,
            dob_auto_now,
            "dob_auto_now",
            now.date(),
            cast_function=lambda x: x.date(),
        )
        dob_auto_now_add = DateField(auto_now_add=True)
        dob_auto_now_add.set_attributes_from_name("dob_auto_now_add")
        self.check_added_field_default(
            editor,
            Author,
            dob_auto_now_add,
            "dob_auto_now_add",
            now.date(),
            cast_function=lambda x: x.date(),
        )
        dtob_auto_now = DateTimeField(auto_now=True)
        dtob_auto_now.set_attributes_from_name("dtob_auto_now")
        self.check_added_field_default(
            editor,
            Author,
            dtob_auto_now,
            "dtob_auto_now",
            now,
        )
        dt_tm_of_birth_auto_now_add = DateTimeField(auto_now_add=True)
        dt_tm_of_birth_auto_now_add.set_attributes_from_name("dtob_auto_now_add")
        self.check_added_field_default(
            editor,
            Author,
            dt_tm_of_birth_auto_now_add,
            "dtob_auto_now_add",
            now,
        )
        tob_auto_now = TimeField(auto_now=True)
        tob_auto_now.set_attributes_from_name("tob_auto_now")
        self.check_added_field_default(
            editor,
            Author,
            tob_auto_now,
            "tob_auto_now",
            now.time(),
            cast_function=lambda x: x.time(),
        )
        tob_auto_now_add = TimeField(auto_now_add=True)
        tob_auto_now_add.set_attributes_from_name("tob_auto_now_add")
        self.check_added_field_default(
            editor,
            Author,
            tob_auto_now_add,
            "tob_auto_now_add",
            now.time(),
            cast_function=lambda x: x.time(),
        )

    def test_namespaced_db_table_create_index_name(self):
        """
        Table names are stripped of their namespace/schema before being used to
        generate index names.
        """
        with connection.schema_editor() as editor:
            max_name_length = connection.ops.max_name_length() or 200
            namespace = "n" * max_name_length
            table_name = "t" * max_name_length
            namespaced_table_name = '"%s"."%s"' % (namespace, table_name)
            self.assertEqual(
                editor._create_index_name(table_name, []),
                editor._create_index_name(namespaced_table_name, []),
            )

    @unittest.skipUnless(
        connection.vendor == "oracle", "Oracle specific db_table syntax"
    )
    def test_creation_with_db_table_double_quotes(self):
        oracle_user = connection.creation._test_database_user()

        class Student(Model):
            name = CharField(max_length=30)

            class Meta:
                app_label = "schema"
                apps = new_apps
                db_table = '"%s"."DJANGO_STUDENT_TABLE"' % oracle_user

        class Document(Model):
            name = CharField(max_length=30)
            students = ManyToManyField(Student)

            class Meta:
                app_label = "schema"
                apps = new_apps
                db_table = '"%s"."DJANGO_DOCUMENT_TABLE"' % oracle_user

        self.isolated_local_models = [Student, Document]

        with connection.schema_editor() as editor:
            editor.create_model(Student)
            editor.create_model(Document)

        doc = Document.objects.create(name="Test Name")
        student = Student.objects.create(name="Some man")
        doc.students.add(student)

    @isolate_apps("schema")
    @unittest.skipUnless(
        connection.vendor == "postgresql", "PostgreSQL specific db_table syntax."
    )
    def test_namespaced_db_table_foreign_key_reference(self):
        with connection.cursor() as cursor:
            cursor.execute("CREATE SCHEMA django_schema_tests")

        def delete_schema():
            with connection.cursor() as cursor:
                cursor.execute("DROP SCHEMA django_schema_tests CASCADE")

        self.addCleanup(delete_schema)

        class Author(Model):
            class Meta:
                app_label = "schema"

        class Book(Model):
            class Meta:
                app_label = "schema"
                db_table = '"django_schema_tests"."schema_book"'

        author = ForeignKey(Author, CASCADE)
        author.set_attributes_from_name("author")

        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
            editor.add_field(Book, author)

    def test_rename_table_renames_deferred_sql_references(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
            editor.alter_db_table(Author, "schema_author", "schema_renamed_author")
            editor.alter_db_table(Author, "schema_book", "schema_renamed_book")
            try:
                self.assertGreater(len(editor.deferred_sql), 0)
                for statement in editor.deferred_sql:
                    self.assertIs(statement.references_table("schema_author"), False)
                    self.assertIs(statement.references_table("schema_book"), False)
            finally:
                editor.alter_db_table(Author, "schema_renamed_author", "schema_author")
                editor.alter_db_table(Author, "schema_renamed_book", "schema_book")

    def test_rename_column_renames_deferred_sql_references(self):
        with connection.schema_editor() as editor:
            editor.create_model(Author)
            editor.create_model(Book)
            old_title = Book._meta.get_field("title")
            new_title = CharField(max_length=100, db_index=True)
            new_title.set_attributes_from_name("renamed_title")
            editor.alter_field(Book, old_title, new_title)
            old_author = Book._meta.get_field("author")
            new_author = ForeignKey(Author, CASCADE)
            new_author.set_attributes_from_name("renamed_author")
            editor.alter_field(Book, old_author, new_author)
            self.assertGreater(len(editor.deferred_sql), 0)
            for statement in editor.deferred_sql:
                self.assertIs(statement.references_column("book", "title"), False)
                self.assertIs(statement.references_column("book", "author_id"), False)

    @isolate_apps("schema")
    def test_referenced_field_without_constraint_rename_inside_atomic_block(self):
        """
        Foreign keys without database level constraint don't prevent the field
        they reference from being renamed in an atomic block.
        """

        class Foo(Model):
            field = CharField(max_length=255, unique=True)

            class Meta:
                app_label = "schema"

        class Bar(Model):
            foo = ForeignKey(Foo, CASCADE, to_field="field", db_constraint=False)

            class Meta:
                app_label = "schema"

        self.isolated_local_models = [Foo, Bar]
        with connection.schema_editor() as editor:
            editor.create_model(Foo)
            editor.create_model(Bar)

        new_field = CharField(max_length=255, unique=True)
        new_field.set_attributes_from_name("renamed")
        with connection.schema_editor(atomic=True) as editor:
            editor.alter_field(Foo, Foo._meta.get_field("field"), new_field)

    @isolate_apps("schema")
    def test_referenced_table_without_constraint_rename_inside_atomic_block(self):
        """
        Foreign keys without database level constraint don't prevent the table
        they reference from being renamed in an atomic block.
        """

        class Foo(Model):
            field = CharField(max_length=255, unique=True)

            class Meta:
                app_label = "schema"

        class Bar(Model):
            foo = ForeignKey(Foo, CASCADE, to_field="field", db_constraint=False)

            class Meta:
                app_label = "schema"

        self.isolated_local_models = [Foo, Bar]
        with connection.schema_editor() as editor:
            editor.create_model(Foo)
            editor.create_model(Bar)

        new_field = CharField(max_length=255, unique=True)
        new_field.set_attributes_from_name("renamed")
        with connection.schema_editor(atomic=True) as editor:
            editor.alter_db_table(Foo, Foo._meta.db_table, "renamed_table")
        Foo._meta.db_table = "renamed_table"

    @isolate_apps("schema")
    @skipUnlessDBFeature("supports_collation_on_charfield")
    def test_db_collation_charfield(self):
        collation = connection.features.test_collations.get("non_default")
        if not collation:
            self.skipTest("Language collations are not supported.")

        class Foo(Model):
            field = CharField(max_length=255, db_collation=collation)

            class Meta:
                app_label = "schema"

        self.isolated_local_models = [Foo]
        with connection.schema_editor() as editor:
            editor.create_model(Foo)

        self.assertEqual(
            self.get_column_collation(Foo._meta.db_table, "field"),
            collation,
        )

    @isolate_apps("schema")
    @skipUnlessDBFeature("supports_collation_on_textfield")
    def test_db_collation_textfield(self):
        collation = connection.features.test_collations.get("non_default")
        if not collation:
            self.skipTest("Language collations are not supported.")

        class Foo(Model):
            field = TextField(db_collation=collation)

            class Meta:
                app_label = "schema"

        self.isolated_local_models = [Foo]
        with connection.schema_editor() as editor:
            editor.create_model(Foo)

        self.assertEqual(
            self.get_column_collation(Foo._meta.db_table, "field"),
            collation,
        )

    @skipUnlessDBFeature("supports_collation_on_charfield")
    def test_add_field_db_collation(self):
        collation = connection.features.test_collations.get("non_default")
        if not collation:
            self.skipTest("Language collations are not supported.")

        with connection.schema_editor() as editor:
            editor.create_model(Author)

        new_field = CharField(max_length=255, db_collation=collation)
        new_field.set_attributes_from_name("alias")
        with connection.schema_editor() as editor:
            editor.add_field(Author, new_field)
        columns = self.column_classes(Author)
        self.assertEqual(
            columns["alias"][0],
            connection.features.introspected_field_types["CharField"],
        )
        self.assertEqual(columns["alias"][1][8], collation)

    @skipUnlessDBFeature("supports_collation_on_charfield")
    def test_alter_field_db_collation(self):
        collation = connection.features.test_collations.get("non_default")
        if not collation:
            self.skipTest("Language collations are not supported.")

        with connection.schema_editor() as editor:
            editor.create_model(Author)

        old_field = Author._meta.get_field("name")
        new_field = CharField(max_length=255, db_collation=collation)
        new_field.set_attributes_from_name("name")
        new_field.model = Author
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        self.assertEqual(
            self.get_column_collation(Author._meta.db_table, "name"),
            collation,
        )
        with connection.schema_editor() as editor:
            editor.alter_field(Author, new_field, old_field, strict=True)
        self.assertIsNone(self.get_column_collation(Author._meta.db_table, "name"))

    @skipUnlessDBFeature("supports_collation_on_charfield")
    def test_alter_field_type_preserve_db_collation(self):
        collation = connection.features.test_collations.get("non_default")
        if not collation:
            self.skipTest("Language collations are not supported.")

        with connection.schema_editor() as editor:
            editor.create_model(Author)

        old_field = Author._meta.get_field("name")
        new_field = CharField(max_length=255, db_collation=collation)
        new_field.set_attributes_from_name("name")
        new_field.model = Author
        with connection.schema_editor() as editor:
            editor.alter_field(Author, old_field, new_field, strict=True)
        self.assertEqual(
            self.get_column_collation(Author._meta.db_table, "name"),
            collation,
        )
        # Changing a field type should preserve the collation.
        old_field = new_field
        new_field = CharField(max_length=511, db_collation=collation)
        new_field.set_attributes_from_name("name")
        new_field.model = Author
        with connection.schema_editor() as editor:
            editor.alter_field(Author, new_field, old_field, strict=True)
        # Collation is preserved.
        self.assertEqual(
            self.get_column_collation(Author._meta.db_table, "name"),
            collation,
        )

    @skipUnlessDBFeature("supports_collation_on_charfield")
    def test_alter_primary_key_db_collation(self):
        collation = connection.features.test_collations.get("non_default")
        if not collation:
            self.skipTest("Language collations are not supported.")

        with connection.schema_editor() as editor:
            editor.create_model(Thing)

        old_field = Thing._meta.get_field("when")
        new_field = CharField(max_length=1, db_collation=collation, primary_key=True)
        new_field.set_attributes_from_name("when")
        new_field.model = Thing
        with connection.schema_editor() as editor:
            editor.alter_field(Thing, old_field, new_field, strict=True)
        self.assertEqual(self.get_primary_key(Thing._meta.db_table), "when")
        self.assertEqual(
            self.get_column_collation(Thing._meta.db_table, "when"),
            collation,
        )
        with connection.schema_editor() as editor:
            editor.alter_field(Thing, new_field, old_field, strict=True)
        self.assertEqual(self.get_primary_key(Thing._meta.db_table), "when")
        self.assertIsNone(self.get_column_collation(Thing._meta.db_table, "when"))

    @skipUnlessDBFeature(
        "supports_collation_on_charfield", "supports_collation_on_textfield"
    )
    def test_alter_field_type_and_db_collation(self):
        collation = connection.features.test_collations.get("non_default")
        if not collation:
            self.skipTest("Language collations are not supported.")

        with connection.schema_editor() as editor:
            editor.create_model(Note)

        old_field = Note._meta.get_field("info")
        new_field = CharField(max_length=255, db_collation=collation)
        new_field.set_attributes_from_name("info")
        new_field.model = Note
        with connection.schema_editor() as editor:
            editor.alter_field(Note, old_field, new_field, strict=True)
        columns = self.column_classes(Note)
        self.assertEqual(
            columns["info"][0],
            connection.features.introspected_field_types["CharField"],
        )
        self.assertEqual(columns["info"][1][8], collation)
        with connection.schema_editor() as editor:
            editor.alter_field(Note, new_field, old_field, strict=True)
        columns = self.column_classes(Note)
        self.assertEqual(columns["info"][0], "TextField")
        self.assertIsNone(columns["info"][1][8])

    @skipUnlessDBFeature(
        "supports_collation_on_charfield",
        "supports_non_deterministic_collations",
    )
    def test_ci_cs_db_collation(self):
        cs_collation = connection.features.test_collations.get("cs")
        ci_collation = connection.features.test_collations.get("ci")
        try:
            if connection.vendor == "mysql":
                cs_collation = "latin1_general_cs"
            elif connection.vendor == "postgresql":
                cs_collation = "en-x-icu"
                with connection.cursor() as cursor:
                    cursor.execute(
                        "CREATE COLLATION IF NOT EXISTS case_insensitive "
                        "(provider = icu, locale = 'und-u-ks-level2', "
                        "deterministic = false)"
                    )
                    ci_collation = "case_insensitive"
            # Create the table.
            with connection.schema_editor() as editor:
                editor.create_model(Author)
            # Case-insensitive collation.
            old_field = Author._meta.get_field("name")
            new_field_ci = CharField(max_length=255, db_collation=ci_collation)
            new_field_ci.set_attributes_from_name("name")
            new_field_ci.model = Author
            with connection.schema_editor() as editor:
                editor.alter_field(Author, old_field, new_field_ci, strict=True)
            Author.objects.create(name="ANDREW")
            self.assertIs(Author.objects.filter(name="Andrew").exists(), True)
            # Case-sensitive collation.
            new_field_cs = CharField(max_length=255, db_collation=cs_collation)
            new_field_cs.set_attributes_from_name("name")
            new_field_cs.model = Author
            with connection.schema_editor() as editor:
                editor.alter_field(Author, new_field_ci, new_field_cs, strict=True)
            self.assertIs(Author.objects.filter(name="Andrew").exists(), False)
        finally:
            if connection.vendor == "postgresql":
                with connection.cursor() as cursor:
                    cursor.execute("DROP COLLATION IF EXISTS case_insensitive")