Fixed #470 -- Added support for database defaults on fields.

Special thanks to Hannes Ljungberg for finding multiple implementation
gaps.

Thanks also to Simon Charette, Adam Johnson, and Mariusz Felisiak for
reviews.

AUTHORS

@@ -587,6 +587,7 @@ answer newbie questions, and generally made Django that much better:
     lerouxb@gmail.com
     Lex Berezhny <lex@damoti.com>
     Liang Feng <hutuworm@gmail.com>
+    Lily Foote
     limodou
     Lincoln Smith <lincoln.smith@anu.edu.au>
     Liu Yijie <007gzs@gmail.com>

django/db/backends/base/features.py

@@ -201,6 +201,15 @@ class BaseDatabaseFeatures:
     # Does the backend require literal defaults, rather than parameterized ones?
     requires_literal_defaults = False
 
+    # Does the backend support functions in defaults?
+    supports_expression_defaults = True
+
+    # Does the backend support the DEFAULT keyword in insert queries?
+    supports_default_keyword_in_insert = True
+
+    # Does the backend support the DEFAULT keyword in bulk insert queries?
+    supports_default_keyword_in_bulk_insert = True
+
     # Does the backend require a connection reset after each material schema change?
     connection_persists_old_columns = False
 
@@ -361,6 +370,9 @@ class BaseDatabaseFeatures:
     # SQL template override for tests.aggregation.tests.NowUTC
     test_now_utc_template = None
 
+    # SQL to create a model instance using the database defaults.
+    insert_test_table_with_defaults = None
+
     # A set of dotted paths to tests in Django's test suite that are expected
     # to fail on this database.
     django_test_expected_failures = set()

django/db/backends/base/schema.py

@@ -12,7 +12,7 @@ from django.db.backends.ddl_references import (
     Table,
 )
 from django.db.backends.utils import names_digest, split_identifier, truncate_name
-from django.db.models import Deferrable, Index
+from django.db.models import NOT_PROVIDED, Deferrable, Index
 from django.db.models.sql import Query
 from django.db.transaction import TransactionManagementError, atomic
 from django.utils import timezone
@@ -296,6 +296,12 @@ class BaseDatabaseSchemaEditor:
             yield self._comment_sql(field.db_comment)
         # Work out nullability.
         null = field.null
+        # Add database default.
+        if field.db_default is not NOT_PROVIDED:
+            default_sql, default_params = self.db_default_sql(field)
+            yield f"DEFAULT {default_sql}"
+            params.extend(default_params)
+            include_default = False
         # Include a default value, if requested.
         include_default = (
             include_default
@@ -400,6 +406,22 @@ class BaseDatabaseSchemaEditor:
         """
         return "%s"
 
+    def db_default_sql(self, field):
+        """Return the sql and params for the field's database default."""
+        from django.db.models.expressions import Value
+
+        sql = "%s" if isinstance(field.db_default, Value) else "(%s)"
+        query = Query(model=field.model)
+        compiler = query.get_compiler(connection=self.connection)
+        default_sql, params = compiler.compile(field.db_default)
+        if self.connection.features.requires_literal_defaults:
+            # Some databases doesn't support parameterized defaults (Oracle,
+            # SQLite). If this is the case, the individual schema backend
+            # should implement prepare_default().
+            default_sql %= tuple(self.prepare_default(p) for p in params)
+            params = []
+        return sql % default_sql, params
+
     @staticmethod
     def _effective_default(field):
         # This method allows testing its logic without a connection.
@@ -1025,6 +1047,21 @@ class BaseDatabaseSchemaEditor:
             )
             actions.append(fragment)
             post_actions.extend(other_actions)
+
+        if new_field.db_default is not NOT_PROVIDED:
+            if (
+                old_field.db_default is NOT_PROVIDED
+                or new_field.db_default != old_field.db_default
+            ):
+                actions.append(
+                    self._alter_column_database_default_sql(model, old_field, new_field)
+                )
+        elif old_field.db_default is not NOT_PROVIDED:
+            actions.append(
+                self._alter_column_database_default_sql(
+                    model, old_field, new_field, drop=True
+                )
+            )
         # When changing a column NULL constraint to NOT NULL with a given
         # default value, we need to perform 4 steps:
         #  1. Add a default for new incoming writes
@@ -1033,7 +1070,11 @@ class BaseDatabaseSchemaEditor:
         #  4. Drop the default again.
         # Default change?
         needs_database_default = False
-        if old_field.null and not new_field.null:
+        if (
+            old_field.null
+            and not new_field.null
+            and new_field.db_default is NOT_PROVIDED
+        ):
             old_default = self.effective_default(old_field)
             new_default = self.effective_default(new_field)
             if (
@@ -1051,9 +1092,9 @@ class BaseDatabaseSchemaEditor:
             if fragment:
                 null_actions.append(fragment)
         # Only if we have a default and there is a change from NULL to NOT NULL
-        four_way_default_alteration = new_field.has_default() and (
-            old_field.null and not new_field.null
-        )
+        four_way_default_alteration = (
+            new_field.has_default() or new_field.db_default is not NOT_PROVIDED
+        ) and (old_field.null and not new_field.null)
         if actions or null_actions:
             if not four_way_default_alteration:
                 # If we don't have to do a 4-way default alteration we can
@@ -1074,15 +1115,20 @@ class BaseDatabaseSchemaEditor:
                     params,
                 )
             if four_way_default_alteration:
+                if new_field.db_default is NOT_PROVIDED:
+                    default_sql = "%s"
+                    params = [new_default]
+                else:
+                    default_sql, params = self.db_default_sql(new_field)
                 # Update existing rows with default value
                 self.execute(
                     self.sql_update_with_default
                     % {
                         "table": self.quote_name(model._meta.db_table),
                         "column": self.quote_name(new_field.column),
-                        "default": "%s",
+                        "default": default_sql,
                     },
-                    [new_default],
+                    params,
                 )
                 # Since we didn't run a NOT NULL change before we need to do it
                 # now
@@ -1264,6 +1310,34 @@ class BaseDatabaseSchemaEditor:
             params,
         )
 
+    def _alter_column_database_default_sql(
+        self, model, old_field, new_field, drop=False
+    ):
+        """
+        Hook to specialize column database default alteration.
+
+        Return a (sql, params) fragment to add or drop (depending on the drop
+        argument) a default to new_field's column.
+        """
+        if drop:
+            sql = self.sql_alter_column_no_default
+            default_sql = ""
+            params = []
+        else:
+            sql = self.sql_alter_column_default
+            default_sql, params = self.db_default_sql(new_field)
+
+        new_db_params = new_field.db_parameters(connection=self.connection)
+        return (
+            sql
+            % {
+                "column": self.quote_name(new_field.column),
+                "type": new_db_params["type"],
+                "default": default_sql,
+            },
+            params,
+        )
+
     def _alter_column_type_sql(
         self, model, old_field, new_field, new_type, old_collation, new_collation
     ):

django/db/backends/mysql/features.py

@@ -51,6 +51,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     # COLLATE must be wrapped in parentheses because MySQL treats COLLATE as an
     # indexed expression.
     collate_as_index_expression = True
+    insert_test_table_with_defaults = "INSERT INTO {} () VALUES ()"
 
     supports_order_by_nulls_modifier = False
     order_by_nulls_first = True
@@ -342,3 +343,9 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         if self.connection.mysql_is_mariadb:
             return self.connection.mysql_version >= (10, 5, 2)
         return True
+
+    @cached_property
+    def supports_expression_defaults(self):
+        if self.connection.mysql_is_mariadb:
+            return True
+        return self.connection.mysql_version >= (8, 0, 13)

django/db/backends/mysql/schema.py

@@ -209,11 +209,15 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         self._create_missing_fk_index(model, fields=fields)
         return super()._delete_composed_index(model, fields, *args)
 
-    def _set_field_new_type_null_status(self, field, new_type):
+    def _set_field_new_type(self, field, new_type):
         """
-        Keep the null property of the old field. If it has changed, it will be
-        handled separately.
+        Keep the NULL and DEFAULT properties of the old field. If it has
+        changed, it will be handled separately.
         """
+        if field.db_default is not NOT_PROVIDED:
+            default_sql, params = self.db_default_sql(field)
+            default_sql %= tuple(self.quote_value(p) for p in params)
+            new_type += f" DEFAULT {default_sql}"
         if field.null:
             new_type += " NULL"
         else:
@@ -223,7 +227,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
     def _alter_column_type_sql(
         self, model, old_field, new_field, new_type, old_collation, new_collation
     ):
-        new_type = self._set_field_new_type_null_status(old_field, new_type)
+        new_type = self._set_field_new_type(old_field, new_type)
         return super()._alter_column_type_sql(
             model, old_field, new_field, new_type, old_collation, new_collation
         )
@@ -242,7 +246,7 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         return field_db_params["check"]
 
     def _rename_field_sql(self, table, old_field, new_field, new_type):
-        new_type = self._set_field_new_type_null_status(old_field, new_type)
+        new_type = self._set_field_new_type(old_field, new_type)
         return super()._rename_field_sql(table, old_field, new_field, new_type)
 
     def _alter_column_comment_sql(self, model, new_field, new_type, new_db_comment):
@@ -252,3 +256,18 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
     def _comment_sql(self, comment):
         comment_sql = super()._comment_sql(comment)
         return f" COMMENT {comment_sql}"
+
+    def _alter_column_null_sql(self, model, old_field, new_field):
+        if new_field.db_default is NOT_PROVIDED:
+            return super()._alter_column_null_sql(model, old_field, new_field)
+
+        new_db_params = new_field.db_parameters(connection=self.connection)
+        type_sql = self._set_field_new_type(new_field, new_db_params["type"])
+        return (
+            "MODIFY %(column)s %(type)s"
+            % {
+                "column": self.quote_name(new_field.column),
+                "type": type_sql,
+            },
+            [],
+        )

django/db/backends/oracle/features.py

@@ -32,6 +32,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     atomic_transactions = False
     nulls_order_largest = True
     requires_literal_defaults = True
+    supports_default_keyword_in_bulk_insert = False
     closed_cursor_error_class = InterfaceError
     bare_select_suffix = " FROM DUAL"
     # Select for update with limit can be achieved on Oracle, but not with the
@@ -130,6 +131,9 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         "annotations.tests.NonAggregateAnnotationTestCase."
         "test_custom_functions_can_ref_other_functions",
     }
+    insert_test_table_with_defaults = (
+        "INSERT INTO {} VALUES (DEFAULT, DEFAULT, DEFAULT)"
+    )
 
     @cached_property
     def introspected_field_types(self):

django/db/backends/oracle/introspection.py

@@ -156,7 +156,7 @@ class DatabaseIntrospection(BaseDatabaseIntrospection):
         field_map = {
             column: (
                 display_size,
-                default if default != "NULL" else None,
+                default.rstrip() if default and default != "NULL" else None,
                 collation,
                 is_autofield,
                 is_json,

django/db/backends/oracle/schema.py

@@ -198,7 +198,9 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         return self.normalize_name(for_name + "_" + suffix)
 
     def prepare_default(self, value):
-        return self.quote_value(value)
+        # Replace % with %% as %-formatting is applied in
+        # FormatStylePlaceholderCursor._fix_for_params().
+        return self.quote_value(value).replace("%", "%%")
 
     def _field_should_be_indexed(self, model, field):
         create_index = super()._field_should_be_indexed(model, field)

django/db/backends/postgresql/features.py

@@ -76,6 +76,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         "swedish_ci": "sv-x-icu",
     }
     test_now_utc_template = "STATEMENT_TIMESTAMP() AT TIME ZONE 'UTC'"
+    insert_test_table_with_defaults = "INSERT INTO {} DEFAULT VALUES"
 
     django_test_skips = {
         "opclasses are PostgreSQL only.": {

django/db/backends/sqlite3/features.py

@@ -59,6 +59,8 @@ class DatabaseFeatures(BaseDatabaseFeatures):
             PRIMARY KEY(column_1, column_2)
         )
     """
+    insert_test_table_with_defaults = 'INSERT INTO {} ("null") VALUES (1)'
+    supports_default_keyword_in_insert = False
 
     @cached_property
     def django_test_skips(self):

django/db/backends/sqlite3/schema.py

@@ -6,7 +6,7 @@ from django.db import NotSupportedError
 from django.db.backends.base.schema import BaseDatabaseSchemaEditor
 from django.db.backends.ddl_references import Statement
 from django.db.backends.utils import strip_quotes
-from django.db.models import UniqueConstraint
+from django.db.models import NOT_PROVIDED, UniqueConstraint
 from django.db.transaction import atomic
 
 
@@ -233,9 +233,13 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
         if create_field:
             body[create_field.name] = create_field
             # Choose a default and insert it into the copy map
-            if not create_field.many_to_many and create_field.concrete:
+            if (
+                create_field.db_default is NOT_PROVIDED
+                and not create_field.many_to_many
+                and create_field.concrete
+            ):
                 mapping[create_field.column] = self.prepare_default(
-                    self.effective_default(create_field),
+                    self.effective_default(create_field)
                 )
         # Add in any altered fields
         for alter_field in alter_fields:
@@ -244,9 +248,13 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
             mapping.pop(old_field.column, None)
             body[new_field.name] = new_field
             if old_field.null and not new_field.null:
+                if new_field.db_default is NOT_PROVIDED:
+                    default = self.prepare_default(self.effective_default(new_field))
+                else:
+                    default, _ = self.db_default_sql(new_field)
                 case_sql = "coalesce(%(col)s, %(default)s)" % {
                     "col": self.quote_name(old_field.column),
-                    "default": self.prepare_default(self.effective_default(new_field)),
+                    "default": default,
                 }
                 mapping[new_field.column] = case_sql
             else:
@@ -381,6 +389,8 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
 
     def add_field(self, model, field):
         """Create a field on a model."""
+        from django.db.models.expressions import Value
+
         # Special-case implicit M2M tables.
         if field.many_to_many and field.remote_field.through._meta.auto_created:
             self.create_model(field.remote_field.through)
@@ -394,6 +404,12 @@ class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):
             # COLUMN statement because DROP DEFAULT is not supported in
             # ALTER TABLE.
             or self.effective_default(field) is not None
+            # Fields with non-constant defaults cannot by handled by ALTER
+            # TABLE ADD COLUMN statement.
+            or (
+                field.db_default is not NOT_PROVIDED
+                and not isinstance(field.db_default, Value)
+            )
         ):
             self._remake_table(model, create_field=field)
         else:

django/db/migrations/autodetector.py

@@ -1040,6 +1040,7 @@ class MigrationAutodetector:
         preserve_default = (
             field.null
             or field.has_default()
+            or field.db_default is not models.NOT_PROVIDED
             or field.many_to_many
             or (field.blank and field.empty_strings_allowed)
             or (isinstance(field, time_fields) and field.auto_now)
@@ -1187,6 +1188,7 @@ class MigrationAutodetector:
                         old_field.null
                         and not new_field.null
                         and not new_field.has_default()
+                        and new_field.db_default is models.NOT_PROVIDED
                         and not new_field.many_to_many
                     ):
                         field = new_field.clone()

django/db/models/base.py

@@ -971,8 +971,10 @@ class Model(AltersData, metaclass=ModelBase):
             not raw
             and not force_insert
             and self._state.adding
-            and meta.pk.default
-            and meta.pk.default is not NOT_PROVIDED
+            and (
+                (meta.pk.default and meta.pk.default is not NOT_PROVIDED)
+                or (meta.pk.db_default and meta.pk.db_default is not NOT_PROVIDED)
+            )
         ):
             force_insert = True
         # If possible, try an UPDATE. If that doesn't update anything, do an INSERT.

django/db/models/expressions.py

@@ -176,6 +176,8 @@ class BaseExpression:
     filterable = True
     # Can the expression can be used as a source expression in Window?
     window_compatible = False
+    # Can the expression be used as a database default value?
+    allowed_default = False
 
     def __init__(self, output_field=None):
         if output_field is not None:
@@ -733,6 +735,10 @@ class CombinedExpression(SQLiteNumericMixin, Expression):
         c.rhs = rhs
         return c
 
+    @cached_property
+    def allowed_default(self):
+        return self.lhs.allowed_default and self.rhs.allowed_default
+
 
 class DurationExpression(CombinedExpression):
     def compile(self, side, compiler, connection):
@@ -804,6 +810,8 @@ class TemporalSubtraction(CombinedExpression):
 class F(Combinable):
     """An object capable of resolving references to existing query objects."""
 
+    allowed_default = False
+
     def __init__(self, name):
         """
         Arguments:
@@ -987,6 +995,10 @@ class Func(SQLiteNumericMixin, Expression):
         copy.extra = self.extra.copy()
         return copy
 
+    @cached_property
+    def allowed_default(self):
+        return all(expression.allowed_default for expression in self.source_expressions)
+
 
 @deconstructible(path="django.db.models.Value")
 class Value(SQLiteNumericMixin, Expression):
@@ -995,6 +1007,7 @@ class Value(SQLiteNumericMixin, Expression):
     # Provide a default value for `for_save` in order to allow unresolved
     # instances to be compiled until a decision is taken in #25425.
     for_save = False
+    allowed_default = True
 
     def __init__(self, value, output_field=None):
         """
@@ -1069,6 +1082,8 @@ class Value(SQLiteNumericMixin, Expression):
 
 
 class RawSQL(Expression):
+    allowed_default = True
+
     def __init__(self, sql, params, output_field=None):
         if output_field is None:
             output_field = fields.Field()
@@ -1110,6 +1125,13 @@ class Star(Expression):
         return "*", []
 
 
+class DatabaseDefault(Expression):
+    """Placeholder expression for the database default in an insert query."""
+
+    def as_sql(self, compiler, connection):
+        return "DEFAULT", []
+
+
 class Col(Expression):
     contains_column_references = True
     possibly_multivalued = False
@@ -1213,6 +1235,7 @@ class ExpressionList(Func):
 
 
 class OrderByList(Func):
+    allowed_default = False
     template = "ORDER BY %(expressions)s"
 
     def __init__(self, *expressions, **extra):
@@ -1270,6 +1293,10 @@ class ExpressionWrapper(SQLiteNumericMixin, Expression):
     def __repr__(self):
         return "{}({})".format(self.__class__.__name__, self.expression)
 
+    @property
+    def allowed_default(self):
+        return self.expression.allowed_default
+
 
 class NegatedExpression(ExpressionWrapper):
     """The logical negation of a conditional expression."""
@@ -1397,6 +1424,10 @@ class When(Expression):
             cols.extend(source.get_group_by_cols())
         return cols
 
+    @cached_property
+    def allowed_default(self):
+        return self.condition.allowed_default and self.result.allowed_default
+
 
 @deconstructible(path="django.db.models.Case")
 class Case(SQLiteNumericMixin, Expression):
@@ -1494,6 +1525,12 @@ class Case(SQLiteNumericMixin, Expression):
             return self.default.get_group_by_cols()
         return super().get_group_by_cols()
 
+    @cached_property
+    def allowed_default(self):
+        return self.default.allowed_default and all(
+            case_.allowed_default for case_ in self.cases
+        )
+
 
 class Subquery(BaseExpression, Combinable):
     """

django/db/models/fields/__init__.py

@@ -202,6 +202,7 @@ class Field(RegisterLookupMixin):
         validators=(),
         error_messages=None,
         db_comment=None,
+        db_default=NOT_PROVIDED,
     ):
         self.name = name
         self.verbose_name = verbose_name  # May be set by set_attributes_from_name
@@ -212,6 +213,13 @@ class Field(RegisterLookupMixin):
         self.remote_field = rel
         self.is_relation = self.remote_field is not None
         self.default = default
+        if db_default is not NOT_PROVIDED and not hasattr(
+            db_default, "resolve_expression"
+        ):
+            from django.db.models.expressions import Value
+
+            db_default = Value(db_default)
+        self.db_default = db_default
         self.editable = editable
         self.serialize = serialize
         self.unique_for_date = unique_for_date
@@ -263,6 +271,7 @@ class Field(RegisterLookupMixin):
         return [
             *self._check_field_name(),
             *self._check_choices(),
+            *self._check_db_default(**kwargs),
             *self._check_db_index(),
             *self._check_db_comment(**kwargs),
             *self._check_null_allowed_for_primary_keys(),
@@ -379,6 +388,39 @@ class Field(RegisterLookupMixin):
             )
         ]
 
+    def _check_db_default(self, databases=None, **kwargs):
+        from django.db.models.expressions import Value
+
+        if (
+            self.db_default is NOT_PROVIDED
+            or isinstance(self.db_default, Value)
+            or databases is None
+        ):
+            return []
+        errors = []
+        for db in databases:
+            if not router.allow_migrate_model(db, self.model):
+                continue
+            connection = connections[db]
+
+            if not getattr(self.db_default, "allowed_default", False) and (
+                connection.features.supports_expression_defaults
+            ):
+                msg = f"{self.db_default} cannot be used in db_default."
+                errors.append(checks.Error(msg, obj=self, id="fields.E012"))
+
+            if not (
+                connection.features.supports_expression_defaults
+                or "supports_expression_defaults"
+                in self.model._meta.required_db_features
+            ):
+                msg = (
+                    f"{connection.display_name} does not support default database "
+                    "values with expressions (db_default)."
+                )
+                errors.append(checks.Error(msg, obj=self, id="fields.E011"))
+        return errors
+
     def _check_db_index(self):
         if self.db_index not in (None, True, False):
             return [
@@ -558,6 +600,7 @@ class Field(RegisterLookupMixin):
             "null": False,
             "db_index": False,
             "default": NOT_PROVIDED,
+            "db_default": NOT_PROVIDED,
             "editable": True,
             "serialize": True,
             "unique_for_date": None,
@@ -876,7 +919,10 @@ class Field(RegisterLookupMixin):
     @property
     def db_returning(self):
         """Private API intended only to be used by Django itself."""
-        return False
+        return (
+            self.db_default is not NOT_PROVIDED
+            and connection.features.can_return_columns_from_insert
+        )
 
     def set_attributes_from_name(self, name):
         self.name = self.name or name
@@ -929,7 +975,13 @@ class Field(RegisterLookupMixin):
 
     def pre_save(self, model_instance, add):
         """Return field's value just before saving."""
-        return getattr(model_instance, self.attname)
+        value = getattr(model_instance, self.attname)
+        if not connection.features.supports_default_keyword_in_insert:
+            from django.db.models.expressions import DatabaseDefault
+
+            if isinstance(value, DatabaseDefault):
+                return self.db_default
+        return value
 
     def get_prep_value(self, value):
         """Perform preliminary non-db specific value checks and conversions."""
@@ -968,6 +1020,11 @@ class Field(RegisterLookupMixin):
                 return self.default
             return lambda: self.default
 
+        if self.db_default is not NOT_PROVIDED:
+            from django.db.models.expressions import DatabaseDefault
+
+            return DatabaseDefault
+
         if (
             not self.empty_strings_allowed
             or self.null

django/db/models/functions/comparison.py

@@ -105,6 +105,7 @@ class Coalesce(Func):
 class Collate(Func):
     function = "COLLATE"
     template = "%(expressions)s %(function)s %(collation)s"
+    allowed_default = False
     # Inspired from
     # https://www.postgresql.org/docs/current/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS
     collation_re = _lazy_re_compile(r"^[\w\-]+$")

django/db/models/lookups.py

@@ -185,6 +185,10 @@ class Lookup(Expression):
             sql = f"CASE WHEN {sql} THEN 1 ELSE 0 END"
         return sql, params
 
+    @cached_property
+    def allowed_default(self):
+        return self.lhs.allowed_default and self.rhs.allowed_default
+
 
 class Transform(RegisterLookupMixin, Func):
     """

django/db/models/query.py

@@ -654,10 +654,19 @@ class QuerySet(AltersData):
         return await sync_to_async(self.create)(**kwargs)
 
     def _prepare_for_bulk_create(self, objs):
+        from django.db.models.expressions import DatabaseDefault
+
+        connection = connections[self.db]
         for obj in objs:
             if obj.pk is None:
                 # Populate new PK values.
                 obj.pk = obj._meta.pk.get_pk_value_on_save(obj)
+            if not connection.features.supports_default_keyword_in_bulk_insert:
+                for field in obj._meta.fields:
+                    value = getattr(obj, field.attname)
+                    if isinstance(value, DatabaseDefault):
+                        setattr(obj, field.attname, field.db_default)
+
             obj._prepare_related_fields_for_save(operation_name="bulk_create")
 
     def _check_bulk_create_options(

docs/ref/checks.txt

@@ -175,6 +175,9 @@ Model fields
   ``choices`` (``<count>`` characters).
 * **fields.E010**: ``<field>`` default should be a callable instead of an
   instance so that it's not shared between all field instances.
+* **fields.E011**: ``<database>`` does not support default database values with
+  expressions (``db_default``).
+* **fields.E012**: ``<expression>`` cannot be used in ``db_default``.
 * **fields.E100**: ``AutoField``\s must set primary_key=True.
 * **fields.E110**: ``BooleanField``\s do not accept null values. *This check
   appeared before support for null values was added in Django 2.1.*

docs/ref/models/expressions.txt

@@ -996,6 +996,13 @@ calling the appropriate methods on the wrapped expression.
 
 .. class:: Expression
 
+    .. attribute:: allowed_default
+
+        .. versionadded:: 5.0
+
+        Tells Django that this expression can be used in
+        :attr:`Field.db_default`. Defaults to ``False``.
+
     .. attribute:: contains_aggregate
 
         Tells Django that this expression contains an aggregate and that a

docs/ref/models/fields.txt

@@ -351,6 +351,38 @@ looking at your Django code. For example::
         db_comment="Date and time when the article was published",
     )
 
+``db_default``
+--------------
+
+.. versionadded:: 5.0
+
+.. attribute:: Field.db_default
+
+The database-computed default value for this field. This can be a literal value
+or a database function, such as :class:`~django.db.models.functions.Now`::
+
+    created = models.DateTimeField(db_default=Now())
+
+More complex expressions can be used, as long as they are made from literals
+and database functions::
+
+    month_due = models.DateField(
+        db_default=TruncMonth(
+            Now() + timedelta(days=90),
+            output_field=models.DateField(),
+        )
+    )
+
+Database defaults cannot reference other fields or models. For example, this is
+invalid::
+
+    end = models.IntegerField(db_default=F("start") + 50)
+
+If both ``db_default`` and :attr:`Field.default` are set, ``default`` will take
+precedence when creating instances in Python code. ``db_default`` will still be
+set at the database level and will be used when inserting rows outside of the
+ORM or when adding a new field in a migration.
+
 ``db_index``
 ------------
 
@@ -408,6 +440,9 @@ The default value is used when new model instances are created and a value
 isn't provided for the field. When the field is a primary key, the default is
 also used when the field is set to ``None``.
 
+The default value can also be set at the database level with
+:attr:`Field.db_default`.
+
 ``editable``
 ------------
 

docs/ref/models/instances.txt

@@ -541,7 +541,8 @@ You may have noticed Django database objects use the same ``save()`` method
 for creating and changing objects. Django abstracts the need to use ``INSERT``
 or ``UPDATE`` SQL statements. Specifically, when you call ``save()`` and the
 object's primary key attribute does **not** define a
-:attr:`~django.db.models.Field.default`, Django follows this algorithm:
+:attr:`~django.db.models.Field.default` or
+:attr:`~django.db.models.Field.db_default`, Django follows this algorithm:
 
 * If the object's primary key attribute is set to a value that evaluates to
   ``True`` (i.e., a value other than ``None`` or the empty string), Django
@@ -551,9 +552,10 @@ object's primary key attribute does **not** define a
   exist in the database), Django executes an ``INSERT``.
 
 If the object's primary key attribute defines a
-:attr:`~django.db.models.Field.default` then Django executes an ``UPDATE`` if
-it is an existing model instance and primary key is set to a value that exists
-in the database. Otherwise, Django executes an ``INSERT``.
+:attr:`~django.db.models.Field.default` or
+:attr:`~django.db.models.Field.db_default` then Django executes an ``UPDATE``
+if it is an existing model instance and primary key is set to a value that
+exists in the database. Otherwise, Django executes an ``INSERT``.
 
 The one gotcha here is that you should be careful not to specify a primary-key
 value explicitly when saving new objects, if you cannot guarantee the
@@ -570,6 +572,10 @@ which returns ``NULL``. In such cases it is possible to revert to the old
 algorithm by setting the :attr:`~django.db.models.Options.select_on_save`
 option to ``True``.
 
+.. versionchanged:: 5.0
+
+    The ``Field.db_default`` parameter was added.
+
 .. _ref-models-force-insert:
 
 Forcing an INSERT or UPDATE

docs/releases/5.0.txt

@@ -108,6 +108,21 @@ Can now be simplified to:
 per-project, per-field, or per-request basis. See
 :ref:`reusable-field-group-templates`.
 
+Database-computed default values
+--------------------------------
+
+The new :attr:`Field.db_default <django.db.models.Field.db_default>` parameter
+sets a database-computed default value. For example::
+
+    from django.db import models
+    from django.db.models.functions import Now, Pi
+
+
+    class MyModel(models.Model):
+        age = models.IntegerField(db_default=18)
+        created = models.DateTimeField(db_default=Now())
+        circumference = models.FloatField(db_default=2 * Pi())
+
 Minor features
 --------------
 
@@ -355,7 +370,16 @@ Database backend API
 This section describes changes that may be needed in third-party database
 backends.
 
-* ...
+* ``DatabaseFeatures.supports_expression_defaults`` should be set to ``False``
+  if the database doesn't support using database functions as defaults.
+
+* ``DatabaseFeatures.supports_default_keyword_in_insert`` should be set to
+  ``False`` if the database doesn't support the ``DEFAULT`` keyword in
+  ``INSERT`` queries.
+
+* ``DatabaseFeatures.supports_default_keyword_in_bulk insert`` should be set to
+  ``False`` if the database doesn't support the ``DEFAULT`` keyword in bulk
+  ``INSERT`` queries.
 
 Using ``create_defaults__exact`` may now be required with ``QuerySet.update_or_create()``
 -----------------------------------------------------------------------------------------

tests/basic/models.py

@@ -49,5 +49,9 @@ class PrimaryKeyWithDefault(models.Model):
     uuid = models.UUIDField(primary_key=True, default=uuid.uuid4)
 
 
+class PrimaryKeyWithDbDefault(models.Model):
+    uuid = models.IntegerField(primary_key=True, db_default=1)
+
+
 class ChildPrimaryKeyWithDefault(PrimaryKeyWithDefault):
     pass

tests/basic/tests.py

@@ -20,6 +20,7 @@ from .models import (
     ArticleSelectOnSave,
     ChildPrimaryKeyWithDefault,
     FeaturedArticle,
+    PrimaryKeyWithDbDefault,
     PrimaryKeyWithDefault,
     SelfRef,
 )
@@ -175,6 +176,11 @@ class ModelInstanceCreationTests(TestCase):
         with self.assertNumQueries(1):
             PrimaryKeyWithDefault().save()
 
+    def test_save_primary_with_db_default(self):
+        # An UPDATE attempt is skipped when a primary key has db_default.
+        with self.assertNumQueries(1):
+            PrimaryKeyWithDbDefault().save()
+
     def test_save_parent_primary_with_default(self):
         # An UPDATE attempt is skipped when an inherited primary key has
         # default.

tests/field_defaults/models.py

@@ -12,6 +12,8 @@ field.
 from datetime import datetime
 
 from django.db import models
+from django.db.models.functions import Coalesce, ExtractYear, Now, Pi
+from django.db.models.lookups import GreaterThan
 
 
 class Article(models.Model):
@@ -20,3 +22,45 @@ class Article(models.Model):
 
     def __str__(self):
         return self.headline
+
+
+class DBArticle(models.Model):
+    """
+    Values or expressions can be passed as the db_default parameter to a field.
+    When the object is created without an explicit value passed in, the
+    database will insert the default value automatically.
+    """
+
+    headline = models.CharField(max_length=100, db_default="Default headline")
+    pub_date = models.DateTimeField(db_default=Now())
+
+    class Meta:
+        required_db_features = {"supports_expression_defaults"}
+
+
+class DBDefaults(models.Model):
+    both = models.IntegerField(default=1, db_default=2)
+    null = models.FloatField(null=True, db_default=1.1)
+
+
+class DBDefaultsFunction(models.Model):
+    number = models.FloatField(db_default=Pi())
+    year = models.IntegerField(db_default=ExtractYear(Now()))
+    added = models.FloatField(db_default=Pi() + 4.5)
+    multiple_subfunctions = models.FloatField(db_default=Coalesce(4.5, Pi()))
+    case_when = models.IntegerField(
+        db_default=models.Case(models.When(GreaterThan(2, 1), then=3), default=4)
+    )
+
+    class Meta:
+        required_db_features = {"supports_expression_defaults"}
+
+
+class DBDefaultsPK(models.Model):
+    language_code = models.CharField(primary_key=True, max_length=2, db_default="en")
+
+
+class DBDefaultsFK(models.Model):
+    language_code = models.ForeignKey(
+        DBDefaultsPK, db_default="fr", on_delete=models.CASCADE
+    )

tests/field_defaults/tests.py

@@ -1,8 +1,28 @@
 from datetime import datetime
+from math import pi
 
-from django.test import TestCase
+from django.db import connection
+from django.db.models import Case, F, FloatField, Value, When
+from django.db.models.expressions import (
+    Expression,
+    ExpressionList,
+    ExpressionWrapper,
+    Func,
+    OrderByList,
+    RawSQL,
+)
+from django.db.models.functions import Collate
+from django.db.models.lookups import GreaterThan
+from django.test import SimpleTestCase, TestCase, skipIfDBFeature, skipUnlessDBFeature
 
-from .models import Article
+from .models import (
+    Article,
+    DBArticle,
+    DBDefaults,
+    DBDefaultsFK,
+    DBDefaultsFunction,
+    DBDefaultsPK,
+)
 
 
 class DefaultTests(TestCase):
@@ -14,3 +34,171 @@ class DefaultTests(TestCase):
         self.assertIsInstance(a.id, int)
         self.assertEqual(a.headline, "Default headline")
         self.assertLess((now - a.pub_date).seconds, 5)
+
+    @skipUnlessDBFeature(
+        "can_return_columns_from_insert", "supports_expression_defaults"
+    )
+    def test_field_db_defaults_returning(self):
+        a = DBArticle()
+        a.save()
+        self.assertIsInstance(a.id, int)
+        self.assertEqual(a.headline, "Default headline")
+        self.assertIsInstance(a.pub_date, datetime)
+
+    @skipIfDBFeature("can_return_columns_from_insert")
+    @skipUnlessDBFeature("supports_expression_defaults")
+    def test_field_db_defaults_refresh(self):
+        a = DBArticle()
+        a.save()
+        a.refresh_from_db()
+        self.assertIsInstance(a.id, int)
+        self.assertEqual(a.headline, "Default headline")
+        self.assertIsInstance(a.pub_date, datetime)
+
+    def test_null_db_default(self):
+        obj1 = DBDefaults.objects.create()
+        if not connection.features.can_return_columns_from_insert:
+            obj1.refresh_from_db()
+        self.assertEqual(obj1.null, 1.1)
+
+        obj2 = DBDefaults.objects.create(null=None)
+        self.assertIsNone(obj2.null)
+
+    @skipUnlessDBFeature("supports_expression_defaults")
+    def test_db_default_function(self):
+        m = DBDefaultsFunction.objects.create()
+        if not connection.features.can_return_columns_from_insert:
+            m.refresh_from_db()
+        self.assertAlmostEqual(m.number, pi)
+        self.assertEqual(m.year, datetime.now().year)
+        self.assertAlmostEqual(m.added, pi + 4.5)
+        self.assertEqual(m.multiple_subfunctions, 4.5)
+
+    @skipUnlessDBFeature("insert_test_table_with_defaults")
+    def test_both_default(self):
+        create_sql = connection.features.insert_test_table_with_defaults
+        with connection.cursor() as cursor:
+            cursor.execute(create_sql.format(DBDefaults._meta.db_table))
+        obj1 = DBDefaults.objects.get()
+        self.assertEqual(obj1.both, 2)
+
+        obj2 = DBDefaults.objects.create()
+        self.assertEqual(obj2.both, 1)
+
+    def test_pk_db_default(self):
+        obj1 = DBDefaultsPK.objects.create()
+        if not connection.features.can_return_columns_from_insert:
+            # refresh_from_db() cannot be used because that needs the pk to
+            # already be known to Django.
+            obj1 = DBDefaultsPK.objects.get(pk="en")
+        self.assertEqual(obj1.pk, "en")
+        self.assertEqual(obj1.language_code, "en")
+
+        obj2 = DBDefaultsPK.objects.create(language_code="de")
+        self.assertEqual(obj2.pk, "de")
+        self.assertEqual(obj2.language_code, "de")
+
+    def test_foreign_key_db_default(self):
+        parent1 = DBDefaultsPK.objects.create(language_code="fr")
+        child1 = DBDefaultsFK.objects.create()
+        if not connection.features.can_return_columns_from_insert:
+            child1.refresh_from_db()
+        self.assertEqual(child1.language_code, parent1)
+
+        parent2 = DBDefaultsPK.objects.create()
+        if not connection.features.can_return_columns_from_insert:
+            # refresh_from_db() cannot be used because that needs the pk to
+            # already be known to Django.
+            parent2 = DBDefaultsPK.objects.get(pk="en")
+        child2 = DBDefaultsFK.objects.create(language_code=parent2)
+        self.assertEqual(child2.language_code, parent2)
+
+    @skipUnlessDBFeature(
+        "can_return_columns_from_insert", "supports_expression_defaults"
+    )
+    def test_case_when_db_default_returning(self):
+        m = DBDefaultsFunction.objects.create()
+        self.assertEqual(m.case_when, 3)
+
+    @skipIfDBFeature("can_return_columns_from_insert")
+    @skipUnlessDBFeature("supports_expression_defaults")
+    def test_case_when_db_default_no_returning(self):
+        m = DBDefaultsFunction.objects.create()
+        m.refresh_from_db()
+        self.assertEqual(m.case_when, 3)
+
+    @skipUnlessDBFeature("supports_expression_defaults")
+    def test_bulk_create_all_db_defaults(self):
+        articles = [DBArticle(), DBArticle()]
+        DBArticle.objects.bulk_create(articles)
+
+        headlines = DBArticle.objects.values_list("headline", flat=True)
+        self.assertSequenceEqual(headlines, ["Default headline", "Default headline"])
+
+    @skipUnlessDBFeature("supports_expression_defaults")
+    def test_bulk_create_all_db_defaults_one_field(self):
+        pub_date = datetime.now()
+        articles = [DBArticle(pub_date=pub_date), DBArticle(pub_date=pub_date)]
+        DBArticle.objects.bulk_create(articles)
+
+        headlines = DBArticle.objects.values_list("headline", "pub_date")
+        self.assertSequenceEqual(
+            headlines,
+            [
+                ("Default headline", pub_date),
+                ("Default headline", pub_date),
+            ],
+        )
+
+    @skipUnlessDBFeature("supports_expression_defaults")
+    def test_bulk_create_mixed_db_defaults(self):
+        articles = [DBArticle(), DBArticle(headline="Something else")]
+        DBArticle.objects.bulk_create(articles)
+
+        headlines = DBArticle.objects.values_list("headline", flat=True)
+        self.assertCountEqual(headlines, ["Default headline", "Something else"])
+
+    @skipUnlessDBFeature("supports_expression_defaults")
+    def test_bulk_create_mixed_db_defaults_function(self):
+        instances = [DBDefaultsFunction(), DBDefaultsFunction(year=2000)]
+        DBDefaultsFunction.objects.bulk_create(instances)
+
+        years = DBDefaultsFunction.objects.values_list("year", flat=True)
+        self.assertCountEqual(years, [2000, datetime.now().year])
+
+
+class AllowedDefaultTests(SimpleTestCase):
+    def test_allowed(self):
+        class Max(Func):
+            function = "MAX"
+
+        tests = [
+            Value(10),
+            Max(1, 2),
+            RawSQL("Now()", ()),
+            Value(10) + Value(7),  # Combined expression.
+            ExpressionList(Value(1), Value(2)),
+            ExpressionWrapper(Value(1), output_field=FloatField()),
+            Case(When(GreaterThan(2, 1), then=3), default=4),
+        ]
+        for expression in tests:
+            with self.subTest(expression=expression):
+                self.assertIs(expression.allowed_default, True)
+
+    def test_disallowed(self):
+        class Max(Func):
+            function = "MAX"
+
+        tests = [
+            Expression(),
+            F("field"),
+            Max(F("count"), 1),
+            Value(10) + F("count"),  # Combined expression.
+            ExpressionList(F("count"), Value(2)),
+            ExpressionWrapper(F("count"), output_field=FloatField()),
+            Collate(Value("John"), "nocase"),
+            OrderByList("field"),
+        ]
+        for expression in tests:
+            with self.subTest(expression=expression):
+                self.assertIs(expression.allowed_default, False)

tests/invalid_models_tests/test_ordinary_fields.py

@@ -4,6 +4,7 @@ import uuid
 from django.core.checks import Error
 from django.core.checks import Warning as DjangoWarning
 from django.db import connection, models
+from django.db.models.functions import Coalesce, Pi
 from django.test import SimpleTestCase, TestCase, skipIfDBFeature, skipUnlessDBFeature
 from django.test.utils import isolate_apps, override_settings
 from django.utils.functional import lazy
@@ -1057,3 +1058,109 @@ class DbCommentTests(TestCase):
 
         errors = Model._meta.get_field("field").check(databases=self.databases)
         self.assertEqual(errors, [])
+
+
+@isolate_apps("invalid_models_tests")
+class InvalidDBDefaultTests(TestCase):
+    def test_db_default(self):
+        class Model(models.Model):
+            field = models.FloatField(db_default=Pi())
+
+        field = Model._meta.get_field("field")
+        errors = field.check(databases=self.databases)
+
+        if connection.features.supports_expression_defaults:
+            expected_errors = []
+        else:
+            msg = (
+                f"{connection.display_name} does not support default database values "
+                "with expressions (db_default)."
+            )
+            expected_errors = [Error(msg=msg, obj=field, id="fields.E011")]
+        self.assertEqual(errors, expected_errors)
+
+    def test_db_default_literal(self):
+        class Model(models.Model):
+            field = models.IntegerField(db_default=1)
+
+        field = Model._meta.get_field("field")
+        errors = field.check(databases=self.databases)
+        self.assertEqual(errors, [])
+
+    def test_db_default_required_db_features(self):
+        class Model(models.Model):
+            field = models.FloatField(db_default=Pi())
+
+            class Meta:
+                required_db_features = {"supports_expression_defaults"}
+
+        field = Model._meta.get_field("field")
+        errors = field.check(databases=self.databases)
+        self.assertEqual(errors, [])
+
+    def test_db_default_expression_invalid(self):
+        expression = models.F("field_name")
+
+        class Model(models.Model):
+            field = models.FloatField(db_default=expression)
+
+        field = Model._meta.get_field("field")
+        errors = field.check(databases=self.databases)
+
+        if connection.features.supports_expression_defaults:
+            msg = f"{expression} cannot be used in db_default."
+            expected_errors = [Error(msg=msg, obj=field, id="fields.E012")]
+        else:
+            msg = (
+                f"{connection.display_name} does not support default database values "
+                "with expressions (db_default)."
+            )
+            expected_errors = [Error(msg=msg, obj=field, id="fields.E011")]
+        self.assertEqual(errors, expected_errors)
+
+    def test_db_default_expression_required_db_features(self):
+        expression = models.F("field_name")
+
+        class Model(models.Model):
+            field = models.FloatField(db_default=expression)
+
+            class Meta:
+                required_db_features = {"supports_expression_defaults"}
+
+        field = Model._meta.get_field("field")
+        errors = field.check(databases=self.databases)
+
+        if connection.features.supports_expression_defaults:
+            msg = f"{expression} cannot be used in db_default."
+            expected_errors = [Error(msg=msg, obj=field, id="fields.E012")]
+        else:
+            expected_errors = []
+        self.assertEqual(errors, expected_errors)
+
+    @skipUnlessDBFeature("supports_expression_defaults")
+    def test_db_default_combined_invalid(self):
+        expression = models.Value(4.5) + models.F("field_name")
+
+        class Model(models.Model):
+            field = models.FloatField(db_default=expression)
+
+        field = Model._meta.get_field("field")
+        errors = field.check(databases=self.databases)
+
+        msg = f"{expression} cannot be used in db_default."
+        expected_error = Error(msg=msg, obj=field, id="fields.E012")
+        self.assertEqual(errors, [expected_error])
+
+    @skipUnlessDBFeature("supports_expression_defaults")
+    def test_db_default_function_arguments_invalid(self):
+        expression = Coalesce(models.Value(4.5), models.F("field_name"))
+
+        class Model(models.Model):
+            field = models.FloatField(db_default=expression)
+
+        field = Model._meta.get_field("field")
+        errors = field.check(databases=self.databases)
+
+        msg = f"{expression} cannot be used in db_default."
+        expected_error = Error(msg=msg, obj=field, id="fields.E012")
+        self.assertEqual(errors, [expected_error])

tests/migrations/test_autodetector.py

@@ -269,6 +269,14 @@ class AutodetectorTests(BaseAutodetectorTests):
             ("name", models.CharField(max_length=200, default="Ada Lovelace")),
         ],
     )
+    author_name_db_default = ModelState(
+        "testapp",
+        "Author",
+        [
+            ("id", models.AutoField(primary_key=True)),
+            ("name", models.CharField(max_length=200, db_default="Ada Lovelace")),
+        ],
+    )
     author_name_check_constraint = ModelState(
         "testapp",
         "Author",
@@ -1289,6 +1297,21 @@ class AutodetectorTests(BaseAutodetectorTests):
         self.assertOperationTypes(changes, "testapp", 0, ["AddField"])
         self.assertOperationAttributes(changes, "testapp", 0, 0, name="name")
 
+    @mock.patch(
+        "django.db.migrations.questioner.MigrationQuestioner.ask_not_null_addition",
+        side_effect=AssertionError("Should not have prompted for not null addition"),
+    )
+    def test_add_not_null_field_with_db_default(self, mocked_ask_method):
+        changes = self.get_changes([self.author_empty], [self.author_name_db_default])
+        self.assertNumberMigrations(changes, "testapp", 1)
+        self.assertOperationTypes(changes, "testapp", 0, ["AddField"])
+        self.assertOperationAttributes(
+            changes, "testapp", 0, 0, name="name", preserve_default=True
+        )
+        self.assertOperationFieldAttributes(
+            changes, "testapp", 0, 0, db_default=models.Value("Ada Lovelace")
+        )
+
     @mock.patch(
         "django.db.migrations.questioner.MigrationQuestioner.ask_not_null_addition",
         side_effect=AssertionError("Should not have prompted for not null addition"),
@@ -1478,6 +1501,23 @@ class AutodetectorTests(BaseAutodetectorTests):
             changes, "testapp", 0, 0, default="Ada Lovelace"
         )
 
+    @mock.patch(
+        "django.db.migrations.questioner.MigrationQuestioner.ask_not_null_alteration",
+        side_effect=AssertionError("Should not have prompted for not null alteration"),
+    )
+    def test_alter_field_to_not_null_with_db_default(self, mocked_ask_method):
+        changes = self.get_changes(
+            [self.author_name_null], [self.author_name_db_default]
+        )
+        self.assertNumberMigrations(changes, "testapp", 1)
+        self.assertOperationTypes(changes, "testapp", 0, ["AlterField"])
+        self.assertOperationAttributes(
+            changes, "testapp", 0, 0, name="name", preserve_default=True
+        )
+        self.assertOperationFieldAttributes(
+            changes, "testapp", 0, 0, db_default=models.Value("Ada Lovelace")
+        )
+
     @mock.patch(
         "django.db.migrations.questioner.MigrationQuestioner.ask_not_null_alteration",
         return_value=models.NOT_PROVIDED,

tests/migrations/test_base.py

@@ -292,6 +292,13 @@ class OperationTestBase(MigrationTestBase):
                     ("id", models.AutoField(primary_key=True)),
                     ("pink", models.IntegerField(default=3)),
                     ("weight", models.FloatField()),
+                    ("green", models.IntegerField(null=True)),
+                    (
+                        "yellow",
+                        models.CharField(
+                            blank=True, null=True, db_default="Yellow", max_length=20
+                        ),
+                    ),
                 ],
                 options=model_options,
             )

tests/migrations/test_operations.py

@@ -1,14 +1,18 @@
+import math
+
 from django.core.exceptions import FieldDoesNotExist
 from django.db import IntegrityError, connection, migrations, models, transaction
 from django.db.migrations.migration import Migration
 from django.db.migrations.operations.fields import FieldOperation
 from django.db.migrations.state import ModelState, ProjectState
-from django.db.models.functions import Abs
+from django.db.models.expressions import Value
+from django.db.models.functions import Abs, Pi
 from django.db.transaction import atomic
 from django.test import (
     SimpleTestCase,
     ignore_warnings,
     override_settings,
+    skipIfDBFeature,
     skipUnlessDBFeature,
 )
 from django.test.utils import CaptureQueriesContext
@@ -1340,7 +1344,7 @@ class OperationTests(OperationTestBase):
         self.assertEqual(operation.describe(), "Add field height to Pony")
         self.assertEqual(operation.migration_name_fragment, "pony_height")
         project_state, new_state = self.make_test_state("test_adfl", operation)
-        self.assertEqual(len(new_state.models["test_adfl", "pony"].fields), 4)
+        self.assertEqual(len(new_state.models["test_adfl", "pony"].fields), 6)
         field = new_state.models["test_adfl", "pony"].fields["height"]
         self.assertEqual(field.default, 5)
         # Test the database alteration
@@ -1528,7 +1532,7 @@ class OperationTests(OperationTestBase):
         )
         new_state = project_state.clone()
         operation.state_forwards("test_adflpd", new_state)
-        self.assertEqual(len(new_state.models["test_adflpd", "pony"].fields), 4)
+        self.assertEqual(len(new_state.models["test_adflpd", "pony"].fields), 6)
         field = new_state.models["test_adflpd", "pony"].fields["height"]
         self.assertEqual(field.default, models.NOT_PROVIDED)
         # Test the database alteration
@@ -1547,6 +1551,169 @@ class OperationTests(OperationTestBase):
             sorted(definition[2]), ["field", "model_name", "name", "preserve_default"]
         )
 
+    def test_add_field_database_default(self):
+        """The AddField operation can set and unset a database default."""
+        app_label = "test_adfldd"
+        table_name = f"{app_label}_pony"
+        project_state = self.set_up_test_model(app_label)
+        operation = migrations.AddField(
+            "Pony", "height", models.FloatField(null=True, db_default=4)
+        )
+        new_state = project_state.clone()
+        operation.state_forwards(app_label, new_state)
+        self.assertEqual(len(new_state.models[app_label, "pony"].fields), 6)
+        field = new_state.models[app_label, "pony"].fields["height"]
+        self.assertEqual(field.default, models.NOT_PROVIDED)
+        self.assertEqual(field.db_default, Value(4))
+        project_state.apps.get_model(app_label, "pony").objects.create(weight=4)
+        self.assertColumnNotExists(table_name, "height")
+        # Add field.
+        with connection.schema_editor() as editor:
+            operation.database_forwards(app_label, editor, project_state, new_state)
+        self.assertColumnExists(table_name, "height")
+        new_model = new_state.apps.get_model(app_label, "pony")
+        old_pony = new_model.objects.get()
+        self.assertEqual(old_pony.height, 4)
+        new_pony = new_model.objects.create(weight=5)
+        if not connection.features.can_return_columns_from_insert:
+            new_pony.refresh_from_db()
+        self.assertEqual(new_pony.height, 4)
+        # Reversal.
+        with connection.schema_editor() as editor:
+            operation.database_backwards(app_label, editor, new_state, project_state)
+        self.assertColumnNotExists(table_name, "height")
+        # Deconstruction.
+        definition = operation.deconstruct()
+        self.assertEqual(definition[0], "AddField")
+        self.assertEqual(definition[1], [])
+        self.assertEqual(
+            definition[2],
+            {
+                "field": field,
+                "model_name": "Pony",
+                "name": "height",
+            },
+        )
+
+    def test_add_field_database_default_special_char_escaping(self):
+        app_label = "test_adflddsce"
+        table_name = f"{app_label}_pony"
+        project_state = self.set_up_test_model(app_label)
+        old_pony_pk = (
+            project_state.apps.get_model(app_label, "pony").objects.create(weight=4).pk
+        )
+        tests = ["%", "'", '"']
+        for db_default in tests:
+            with self.subTest(db_default=db_default):
+                operation = migrations.AddField(
+                    "Pony",
+                    "special_char",
+                    models.CharField(max_length=1, db_default=db_default),
+                )
+                new_state = project_state.clone()
+                operation.state_forwards(app_label, new_state)
+                self.assertEqual(len(new_state.models[app_label, "pony"].fields), 6)
+                field = new_state.models[app_label, "pony"].fields["special_char"]
+                self.assertEqual(field.default, models.NOT_PROVIDED)
+                self.assertEqual(field.db_default, Value(db_default))
+                self.assertColumnNotExists(table_name, "special_char")
+                with connection.schema_editor() as editor:
+                    operation.database_forwards(
+                        app_label, editor, project_state, new_state
+                    )
+                self.assertColumnExists(table_name, "special_char")
+                new_model = new_state.apps.get_model(app_label, "pony")
+                try:
+                    new_pony = new_model.objects.create(weight=5)
+                    if not connection.features.can_return_columns_from_insert:
+                        new_pony.refresh_from_db()
+                    self.assertEqual(new_pony.special_char, db_default)
+
+                    old_pony = new_model.objects.get(pk=old_pony_pk)
+                    if connection.vendor != "oracle" or db_default != "'":
+                        # The single quotation mark ' is properly quoted and is
+                        # set for new rows on Oracle, however it is not set on
+                        # existing rows. Skip the assertion as it's probably a
+                        # bug in Oracle.
+                        self.assertEqual(old_pony.special_char, db_default)
+                finally:
+                    with connection.schema_editor() as editor:
+                        operation.database_backwards(
+                            app_label, editor, new_state, project_state
+                        )
+
+    @skipUnlessDBFeature("supports_expression_defaults")
+    def test_add_field_database_default_function(self):
+        app_label = "test_adflddf"
+        table_name = f"{app_label}_pony"
+        project_state = self.set_up_test_model(app_label)
+        operation = migrations.AddField(
+            "Pony", "height", models.FloatField(db_default=Pi())
+        )
+        new_state = project_state.clone()
+        operation.state_forwards(app_label, new_state)
+        self.assertEqual(len(new_state.models[app_label, "pony"].fields), 6)
+        field = new_state.models[app_label, "pony"].fields["height"]
+        self.assertEqual(field.default, models.NOT_PROVIDED)
+        self.assertEqual(field.db_default, Pi())
+        project_state.apps.get_model(app_label, "pony").objects.create(weight=4)
+        self.assertColumnNotExists(table_name, "height")
+        # Add field.
+        with connection.schema_editor() as editor:
+            operation.database_forwards(app_label, editor, project_state, new_state)
+        self.assertColumnExists(table_name, "height")
+        new_model = new_state.apps.get_model(app_label, "pony")
+        old_pony = new_model.objects.get()
+        self.assertAlmostEqual(old_pony.height, math.pi)
+        new_pony = new_model.objects.create(weight=5)
+        if not connection.features.can_return_columns_from_insert:
+            new_pony.refresh_from_db()
+        self.assertAlmostEqual(old_pony.height, math.pi)
+
+    def test_add_field_both_defaults(self):
+        """The AddField operation with both default and db_default."""
+        app_label = "test_adflbddd"
+        table_name = f"{app_label}_pony"
+        project_state = self.set_up_test_model(app_label)
+        operation = migrations.AddField(
+            "Pony", "height", models.FloatField(default=3, db_default=4)
+        )
+        new_state = project_state.clone()
+        operation.state_forwards(app_label, new_state)
+        self.assertEqual(len(new_state.models[app_label, "pony"].fields), 6)
+        field = new_state.models[app_label, "pony"].fields["height"]
+        self.assertEqual(field.default, 3)
+        self.assertEqual(field.db_default, Value(4))
+        project_state.apps.get_model(app_label, "pony").objects.create(weight=4)
+        self.assertColumnNotExists(table_name, "height")
+        # Add field.
+        with connection.schema_editor() as editor:
+            operation.database_forwards(app_label, editor, project_state, new_state)
+        self.assertColumnExists(table_name, "height")
+        new_model = new_state.apps.get_model(app_label, "pony")
+        old_pony = new_model.objects.get()
+        self.assertEqual(old_pony.height, 4)
+        new_pony = new_model.objects.create(weight=5)
+        if not connection.features.can_return_columns_from_insert:
+            new_pony.refresh_from_db()
+        self.assertEqual(new_pony.height, 3)
+        # Reversal.
+        with connection.schema_editor() as editor:
+            operation.database_backwards(app_label, editor, new_state, project_state)
+        self.assertColumnNotExists(table_name, "height")
+        # Deconstruction.
+        definition = operation.deconstruct()
+        self.assertEqual(definition[0], "AddField")
+        self.assertEqual(definition[1], [])
+        self.assertEqual(
+            definition[2],
+            {
+                "field": field,
+                "model_name": "Pony",
+                "name": "height",
+            },
+        )
+
     def test_add_field_m2m(self):
         """
         Tests the AddField operation with a ManyToManyField.
@@ -1558,7 +1725,7 @@ class OperationTests(OperationTestBase):
         )
         new_state = project_state.clone()
         operation.state_forwards("test_adflmm", new_state)
-        self.assertEqual(len(new_state.models["test_adflmm", "pony"].fields), 4)
+        self.assertEqual(len(new_state.models["test_adflmm", "pony"].fields), 6)
         # Test the database alteration
         self.assertTableNotExists("test_adflmm_pony_stables")
         with connection.schema_editor() as editor:
@@ -1727,7 +1894,7 @@ class OperationTests(OperationTestBase):
         self.assertEqual(operation.migration_name_fragment, "remove_pony_pink")
         new_state = project_state.clone()
         operation.state_forwards("test_rmfl", new_state)
-        self.assertEqual(len(new_state.models["test_rmfl", "pony"].fields), 2)
+        self.assertEqual(len(new_state.models["test_rmfl", "pony"].fields), 4)
         # Test the database alteration
         self.assertColumnExists("test_rmfl_pony", "pink")
         with connection.schema_editor() as editor:
@@ -1934,6 +2101,146 @@ class OperationTests(OperationTestBase):
         self.assertEqual(definition[1], [])
         self.assertEqual(sorted(definition[2]), ["field", "model_name", "name"])
 
+    def test_alter_field_add_database_default(self):
+        app_label = "test_alfladd"
+        project_state = self.set_up_test_model(app_label)
+        operation = migrations.AlterField(
+            "Pony", "weight", models.FloatField(db_default=4.5)
+        )
+        new_state = project_state.clone()
+        operation.state_forwards(app_label, new_state)
+        old_weight = project_state.models[app_label, "pony"].fields["weight"]
+        self.assertIs(old_weight.db_default, models.NOT_PROVIDED)
+        new_weight = new_state.models[app_label, "pony"].fields["weight"]
+        self.assertEqual(new_weight.db_default, Value(4.5))
+        with self.assertRaises(IntegrityError), transaction.atomic():
+            project_state.apps.get_model(app_label, "pony").objects.create()
+        # Alter field.
+        with connection.schema_editor() as editor:
+            operation.database_forwards(app_label, editor, project_state, new_state)
+        pony = new_state.apps.get_model(app_label, "pony").objects.create()
+        if not connection.features.can_return_columns_from_insert:
+            pony.refresh_from_db()
+        self.assertEqual(pony.weight, 4.5)
+        # Reversal.
+        with connection.schema_editor() as editor:
+            operation.database_backwards(app_label, editor, new_state, project_state)
+        with self.assertRaises(IntegrityError), transaction.atomic():
+            project_state.apps.get_model(app_label, "pony").objects.create()
+        # Deconstruction.
+        definition = operation.deconstruct()
+        self.assertEqual(definition[0], "AlterField")
+        self.assertEqual(definition[1], [])
+        self.assertEqual(
+            definition[2],
+            {
+                "field": new_weight,
+                "model_name": "Pony",
+                "name": "weight",
+            },
+        )
+
+    def test_alter_field_change_default_to_database_default(self):
+        """The AlterField operation changing default to db_default."""
+        app_label = "test_alflcdtdd"
+        project_state = self.set_up_test_model(app_label)
+        operation = migrations.AlterField(
+            "Pony", "pink", models.IntegerField(db_default=4)
+        )
+        new_state = project_state.clone()
+        operation.state_forwards(app_label, new_state)
+        old_pink = project_state.models[app_label, "pony"].fields["pink"]
+        self.assertEqual(old_pink.default, 3)
+        self.assertIs(old_pink.db_default, models.NOT_PROVIDED)
+        new_pink = new_state.models[app_label, "pony"].fields["pink"]
+        self.assertIs(new_pink.default, models.NOT_PROVIDED)
+        self.assertEqual(new_pink.db_default, Value(4))
+        pony = project_state.apps.get_model(app_label, "pony").objects.create(weight=1)
+        self.assertEqual(pony.pink, 3)
+        # Alter field.
+        with connection.schema_editor() as editor:
+            operation.database_forwards(app_label, editor, project_state, new_state)
+        pony = new_state.apps.get_model(app_label, "pony").objects.create(weight=1)
+        if not connection.features.can_return_columns_from_insert:
+            pony.refresh_from_db()
+        self.assertEqual(pony.pink, 4)
+        # Reversal.
+        with connection.schema_editor() as editor:
+            operation.database_backwards(app_label, editor, new_state, project_state)
+        pony = project_state.apps.get_model(app_label, "pony").objects.create(weight=1)
+        self.assertEqual(pony.pink, 3)
+
+    def test_alter_field_change_nullable_to_database_default_not_null(self):
+        """
+        The AlterField operation changing a null field to db_default.
+        """
+        app_label = "test_alflcntddnn"
+        project_state = self.set_up_test_model(app_label)
+        operation = migrations.AlterField(
+            "Pony", "green", models.IntegerField(db_default=4)
+        )
+        new_state = project_state.clone()
+        operation.state_forwards(app_label, new_state)
+        old_green = project_state.models[app_label, "pony"].fields["green"]
+        self.assertIs(old_green.db_default, models.NOT_PROVIDED)
+        new_green = new_state.models[app_label, "pony"].fields["green"]
+        self.assertEqual(new_green.db_default, Value(4))
+        old_pony = project_state.apps.get_model(app_label, "pony").objects.create(
+            weight=1
+        )
+        self.assertIsNone(old_pony.green)
+        # Alter field.
+        with connection.schema_editor() as editor:
+            operation.database_forwards(app_label, editor, project_state, new_state)
+        old_pony.refresh_from_db()
+        self.assertEqual(old_pony.green, 4)
+        pony = new_state.apps.get_model(app_label, "pony").objects.create(weight=1)
+        if not connection.features.can_return_columns_from_insert:
+            pony.refresh_from_db()
+        self.assertEqual(pony.green, 4)
+        # Reversal.
+        with connection.schema_editor() as editor:
+            operation.database_backwards(app_label, editor, new_state, project_state)
+        pony = project_state.apps.get_model(app_label, "pony").objects.create(weight=1)
+        self.assertIsNone(pony.green)
+
+    @skipIfDBFeature("interprets_empty_strings_as_nulls")
+    def test_alter_field_change_blank_nullable_database_default_to_not_null(self):
+        app_label = "test_alflcbnddnn"
+        table_name = f"{app_label}_pony"
+        project_state = self.set_up_test_model(app_label)
+        default = "Yellow"
+        operation = migrations.AlterField(
+            "Pony",
+            "yellow",
+            models.CharField(blank=True, db_default=default, max_length=20),
+        )
+        new_state = project_state.clone()
+        operation.state_forwards(app_label, new_state)
+        self.assertColumnNull(table_name, "yellow")
+        pony = project_state.apps.get_model(app_label, "pony").objects.create(
+            weight=1, yellow=None
+        )
+        self.assertIsNone(pony.yellow)
+        # Alter field.
+        with connection.schema_editor() as editor:
+            operation.database_forwards(app_label, editor, project_state, new_state)
+        self.assertColumnNotNull(table_name, "yellow")
+        pony.refresh_from_db()
+        self.assertEqual(pony.yellow, default)
+        pony = new_state.apps.get_model(app_label, "pony").objects.create(weight=1)
+        if not connection.features.can_return_columns_from_insert:
+            pony.refresh_from_db()
+        self.assertEqual(pony.yellow, default)
+        # Reversal.
+        with connection.schema_editor() as editor:
+            operation.database_backwards(app_label, editor, new_state, project_state)
+        self.assertColumnNull(table_name, "yellow")
+        pony = project_state.apps.get_model(app_label, "pony").objects.create(
+            weight=1, yellow=None
+        )
+        self.assertIsNone(pony.yellow)
+
     def test_alter_field_add_db_column_noop(self):
         """
         AlterField operation is a noop when adding only a db_column and the

tests/schema/tests.py

@@ -2102,6 +2102,33 @@ class SchemaTests(TransactionTestCase):
         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(
+            atomic=connection.features.supports_atomic_references_rename
+        ) 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")
+
     @skipUnlessDBFeature(
         "supports_column_check_constraints", "can_introspect_check_constraints"
     )