[5.0.x] Fixed #35149 -- Fixed crashes of db_default with unresolvable output field.

Field.db_default accepts either literal Python values or compilables
(as_sql) and wrap the former ones in Value internally.

While 1e38f11 added support for automatic resolving of output fields for
types such as str, int, float, and other unambigous ones it's cannot do
so for all types such as dict or even contrib.postgres and contrib.gis
primitives.

When a literal, non-compilable, value is provided it likely make the
most sense to bind its output field to the field its attached to avoid
forcing the user to provide an explicit `Value(output_field)`.

Thanks David Sanders for the report.

Backport of e67d7d70fa10c06aca36b9057f82054eda45269d from main

django/db/backends/base/schema.py

@@ -412,14 +412,13 @@ class BaseDatabaseSchemaEditor:
         """Return the sql and params for the field's database default."""
         from django.db.models.expressions import Value
 
+        db_default = field._db_default_expression
         sql = (
-            self._column_default_sql(field)
-            if isinstance(field.db_default, Value)
-            else "(%s)"
+            self._column_default_sql(field) if isinstance(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)
+        default_sql, params = compiler.compile(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

django/db/models/fields/__init__.py

@@ -218,12 +218,6 @@ 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
@@ -407,7 +401,7 @@ class Field(RegisterLookupMixin):
                 continue
             connection = connections[db]
 
-            if not getattr(self.db_default, "allowed_default", False) and (
+            if not getattr(self._db_default_expression, "allowed_default", False) and (
                 connection.features.supports_expression_defaults
             ):
                 msg = f"{self.db_default} cannot be used in db_default."
@@ -993,7 +987,7 @@ class Field(RegisterLookupMixin):
             from django.db.models.expressions import DatabaseDefault
 
             if isinstance(value, DatabaseDefault):
-                return self.db_default
+                return self._db_default_expression
         return value
 
     def get_prep_value(self, value):
@@ -1046,6 +1040,17 @@ class Field(RegisterLookupMixin):
             return return_None
         return str  # return empty string
 
+    @cached_property
+    def _db_default_expression(self):
+        db_default = self.db_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)
+        return db_default
+
     def get_choices(
         self,
         include_blank=True,

docs/releases/5.0.2.txt

@@ -36,3 +36,8 @@ Bugfixes
 * Fixed a bug in Django 5.0 that caused a migration crash on MySQL when adding
   a ``BinaryField``, ``TextField``, ``JSONField``, or ``GeometryField`` with a
   ``db_default`` (:ticket:`35162`).
+
+* Fixed a bug in Django 5.0 that caused a migration crash on models with a
+  literal ``db_default`` of a complex type such as ``dict`` instance of a
+  ``JSONField``. Running ``makemigrations`` might generate no-op ``AlterField``
+  operations for fields using ``db_default`` (:ticket:`35149`).

tests/migrations/test_autodetector.py

@@ -1310,7 +1310,7 @@ class AutodetectorTests(BaseAutodetectorTests):
             changes, "testapp", 0, 0, name="name", preserve_default=True
         )
         self.assertOperationFieldAttributes(
-            changes, "testapp", 0, 0, db_default=models.Value("Ada Lovelace")
+            changes, "testapp", 0, 0, db_default="Ada Lovelace"
         )
 
     @mock.patch(
@@ -1516,7 +1516,7 @@ class AutodetectorTests(BaseAutodetectorTests):
             changes, "testapp", 0, 0, name="name", preserve_default=True
         )
         self.assertOperationFieldAttributes(
-            changes, "testapp", 0, 0, db_default=models.Value("Ada Lovelace")
+            changes, "testapp", 0, 0, db_default="Ada Lovelace"
         )
 
     @mock.patch(

tests/migrations/test_operations.py

@@ -1574,7 +1574,7 @@ class OperationTests(OperationTestBase):
         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))
+        self.assertEqual(field.db_default, 4)
         project_state.apps.get_model(app_label, "pony").objects.create(weight=4)
         self.assertColumnNotExists(table_name, "height")
         # Add field.
@@ -1625,7 +1625,7 @@ class OperationTests(OperationTestBase):
                 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.assertEqual(field.db_default, db_default)
                 self.assertColumnNotExists(table_name, "special_char")
                 with connection.schema_editor() as editor:
                     operation.database_forwards(
@@ -1693,7 +1693,7 @@ class OperationTests(OperationTestBase):
         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))
+        self.assertEqual(field.db_default, 4)
         pre_pony_pk = (
             project_state.apps.get_model(app_label, "pony").objects.create(weight=4).pk
         )
@@ -2128,7 +2128,7 @@ class OperationTests(OperationTestBase):
         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))
+        self.assertEqual(new_weight.db_default, 4.5)
         with self.assertRaises(IntegrityError), transaction.atomic():
             project_state.apps.get_model(app_label, "pony").objects.create()
         # Alter field.
@@ -2170,7 +2170,7 @@ class OperationTests(OperationTestBase):
         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))
+        self.assertEqual(new_pink.db_default, 4)
         pony = project_state.apps.get_model(app_label, "pony").objects.create(weight=1)
         self.assertEqual(pony.pink, 3)
         # Alter field.
@@ -2200,7 +2200,7 @@ class OperationTests(OperationTestBase):
         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))
+        self.assertEqual(new_green.db_default, 4)
         old_pony = project_state.apps.get_model(app_label, "pony").objects.create(
             weight=1
         )

tests/schema/tests.py

@@ -7,6 +7,7 @@ from unittest import mock
 
 from django.core.exceptions import FieldError
 from django.core.management.color import no_style
+from django.core.serializers.json import DjangoJSONEncoder
 from django.db import (
     DatabaseError,
     DataError,
@@ -2336,6 +2337,26 @@ class SchemaTests(TransactionTestCase):
         with connection.schema_editor() as editor, self.assertNumQueries(0):
             editor.alter_field(Author, Author._meta.get_field("name"), new_field)
 
+    @isolate_apps("schema")
+    def test_db_default_output_field_resolving(self):
+        class Author(Model):
+            data = JSONField(
+                encoder=DjangoJSONEncoder,
+                db_default={
+                    "epoch": datetime.datetime(1970, 1, 1, tzinfo=datetime.timezone.utc)
+                },
+            )
+
+            class Meta:
+                app_label = "schema"
+
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+
+        author = Author.objects.create()
+        author.refresh_from_db()
+        self.assertEqual(author.data, {"epoch": "1970-01-01T00:00:00Z"})
+
     @skipUnlessDBFeature(
         "supports_column_check_constraints", "can_introspect_check_constraints"
     )