Issues with model Manager and inheritance.
|Reported by:||loic84||Owned by:||nobody|
|Component:||Database layer (models, ORM)||Version:||master|
|Cc:||Anssi Kääriäinen, LaurensBER, aronp@…, Marc Tamlyn||Triage Stage:||Ready for checkin|
|Has patch:||yes||Needs documentation:||no|
|Needs tests:||no||Patch needs improvement:||no|
Description (last modified by )
I've found a couple of issues with the current handling of managers with inheritance.
class AbstractEvent(models.Model): events = models.Manager() class Meta: abstract = True class BachelorParty(AbstractEvent): objects = models.Manager() class MessyBachelorParty(BachelorParty): pass
AbstractEvent has a manager called anything but
MessyBachelorParty.objects.model == <class 'model_inheritance_regress.models.BachelorParty'>
Which causes the following failure:
model_inheritance_regress.tests.ModelInheritanceTest.test_abstract_base_class_m2m_relation_inheritance` fails with `AttributeError: 'BachelorParty' object has no attribute 'bachelorparty_ptr'.
AbstractEvent doesn't have an explicit manager, or has one called
MessyBachelorParty.objects.model == <class 'model_inheritance_regress.models.MessyBachelorParty'> which is the expected result.
- Managers in MTI *are* inherited, contrary to what is said in the docs (now tracked specifically in #25897):
"Managers defined on non-abstract base classes are not inherited by child classes. [...] Therefore, they aren’t passed onto child classes.".
The problem is that, since we create the new class by calling
object.__new__(), normal Python attribute resolution applies, and we gain access to the attributes of the base classes. This doesn't happen to fields because these are removed during the class creation process, but managers are left behind. It's always tempting to think we could just delete them, but you cannot delete something that is not in your own
__dict__. The problem is not so much that we inherit those managers, but that they don't return the right model type as demonstrated in https://github.com/loic/django/compare/manager_inheritance_bug2.
Change History (23)
comment:1 Changed 3 years ago by
|Patch needs improvement:||set|
comment:21 Changed 5 months ago by
|Patch needs improvement:||unset|
|Triage Stage:||Accepted → Ready for checkin|