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.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, skipIfDBFeature, skipUnlessDBFeature
from django.test.utils import CaptureQueriesContext, isolate_apps, register_lookup

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_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")

    @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_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)

    @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))

    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,
        )

    @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")
    @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")