[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 e67d7d7 from main

django/db/backends/base/schema.py

@@ -412,14 +412,13 @@ def db_default_sql(self, field):
         """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 @@ def __init__(
         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 @@ def _check_db_default(self, databases=None, **kwargs):
                 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 @@ def pre_save(self, model_instance, add):
             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 @@ def _get_default(self):
             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 @@ def test_add_not_null_field_with_db_default(self, mocked_ask_method):
             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 @@ def test_alter_field_to_not_null_with_db_default(self, mocked_ask_method):
             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 @@ def test_add_field_database_default(self):
         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 @@ def test_add_field_database_default_special_char_escaping(self):
                 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 @@ def test_add_field_both_defaults(self):
         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 @@ def test_alter_field_add_database_default(self):
         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 @@ def test_alter_field_change_default_to_database_default(self):
         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 @@ def test_alter_field_change_nullable_to_database_default_not_null(self):
         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 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 Meta:
         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"
     )