[5.0.x] Fixed #34946 -- Preserved db_default on combined default field addition.

Regression in 7414704e88.
Backport of 8a28e983df from main

django/db/backends/base/schema.py

@@ -745,9 +745,9 @@ class BaseDatabaseSchemaEditor:
         }
         self.execute(sql, params)
         # Drop the default if we need to
-        # (Django usually does not use in-database defaults)
         if (
-            not self.skip_default_on_alter(field)
+            field.db_default is NOT_PROVIDED
+            and not self.skip_default_on_alter(field)
             and self.effective_default(field) is not None
         ):
             changes_sql, params = self._alter_column_default_sql(

tests/migrations/test_operations.py

@@ -1693,15 +1693,22 @@ class OperationTests(OperationTestBase):
         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)
+        pre_pony_pk = (
+            project_state.apps.get_model(app_label, "pony").objects.create(weight=4).pk
+        )
         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")
+        post_pony_pk = (
+            project_state.apps.get_model(app_label, "pony").objects.create(weight=10).pk
+        )
         new_model = new_state.apps.get_model(app_label, "pony")
-        old_pony = new_model.objects.get()
-        self.assertEqual(old_pony.height, 4)
+        pre_pony = new_model.objects.get(pk=pre_pony_pk)
+        self.assertEqual(pre_pony.height, 4)
+        post_pony = new_model.objects.get(pk=post_pony_pk)
+        self.assertEqual(post_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()

tests/schema/tests.py

@@ -2263,6 +2263,23 @@ class SchemaTests(TransactionTestCase):
         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"
     )