Add corner-case test for model_update

- When only updating m2m field, don't bump `updated_at`

Author: RadoRado

styleguide_example/common/services.py

@@ -32,6 +32,7 @@ def user_update(*, user: User, data) -> User:
         - If something in present in `fields` but not present in `data`, we simply skip.
         - There's a strict assertion that all values in `fields` are actual fields in `instance`.
         - `fields` can support m2m fields, which are handled after the update on `instance`.
+        - If `auto_updated_at` is True, we'll try bumping `updated_at` with the current timestmap.
     """
     has_updated = False
     m2m_data = {}
@@ -62,7 +63,7 @@ def user_update(*, user: User, data) -> User:
     # Perform an update only if any of the fields were actually changed
     if has_updated:
         if auto_updated_at:
-            # We want to take care of the updated_at field,
+            # We want to take care of the `updated_at` field,
             # Only if the models has that field
             # And if no value for updated_at has been provided
             if "updated_at" in model_fields and "updated_at" not in update_fields:

styleguide_example/common/tests/services/test_model_update.py

@@ -80,12 +80,19 @@ def test_model_update_updates_many_to_many_fields(self):
 
         self.assertNotIn(simple_obj, instance.simple_objects.all())
 
+        original_updated_at = instance.updated_at
+
         updated_instance, has_updated = model_update(instance=instance, fields=update_fields, data=data)
 
         self.assertEqual(updated_instance, instance)
         self.assertTrue(has_updated)
 
         self.assertIn(simple_obj, updated_instance.simple_objects.all())
+        self.assertEqual(
+            original_updated_at,
+            updated_instance.updated_at,
+            "If we are only updating m2m fields, don't auto-bump `updated_at`",
+        )
 
     def test_model_update_updates_standard_and_many_to_many_fields(self):
         instance = RandomModelFactory()