Fixed #22581: Pass default values for schema through get_db_prep_save()

tests/schema/tests.py

@@ -231,6 +231,40 @@ class SchemaTests(TransactionTestCase):
         else:
             self.assertEqual(field_type, 'BooleanField')
 
+    def test_add_field_default_transform(self):
+        """
+        Tests adding fields to models with a default that is not directly
+        valid in the database (#22581)
+        """
+        class TestTransformField(IntegerField):
+            # Weird field that saves the count of items in its value
+            def get_default(self):
+                return self.default
+            def get_prep_value(self, value):
+                if value is None:
+                    return 0
+                return len(value)
+        # Create the table
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+        # Add some rows of data
+        Author.objects.create(name="Andrew", height=30)
+        Author.objects.create(name="Andrea")
+        # Add the field with a default it needs to cast (to string in this case)
+        new_field = TestTransformField(default={1:2})
+        new_field.set_attributes_from_name("thing")
+        with connection.schema_editor() as editor:
+            editor.add_field(
+                Author,
+                new_field,
+            )
+        # Ensure the field is there
+        columns = self.column_classes(Author)
+        field_type, field_info = columns['thing']
+        self.assertEqual(field_type, 'IntegerField')
+        # Make sure the values were transformed correctly
+        self.assertEqual(Author.objects.extra(where=["thing = 1"]).count(), 2)
+
     def test_alter(self):
         """
         Tests simple altering of fields