Model.objects cannot be defined using QuerySet.as_manager

[orsinium]

Bug report

What's wrong

class SubscriptionPromotion(BasePromotion):
    objects = SubscriptionPromotionQuerySet.as_manager()

error: Cannot override class variable (previously declared on base class "BasePromotion") with instance variable  [misc]
error: Incompatible types in assignment (expression has type "Manager[BasePromotion]", base class "BasePromotion" defined the type as "ManagerFromPromotionQuerySet[BasePromotion]")  [assignment]

How is that should be

The second error was there before but the first one is newly introduced in 4.2.4. As suggested in #1712 (comment), this code should be explicitly annotated. However, the problem is that if I annotate it like this:

objects: ClassVar[models.Manager[BasePromotion]

Then all custom attributes defined in the SubscriptionPromotionQuerySet won't be available in SubscriptionPromotion.objects.

In other words, before django-stubs-ext used to construct on the fly the return type of QuerySet.as_manager as a mix of Manager and custom methods but now I can't use it because I have to add explicit annotations, and there is no explicit annotation for this dynamically constructed type.

System information

• OS: linux
• python version: 3.8
• django version: 4.2.4
• mypy version: 1.5.1
• django-stubs version: 4.2.4
• django-stubs-ext version: 4.2.2

[orsinium]

We've been using django-stubs for more than a year, and that's the hardest upgrade we had so far, TBH. Automatic inference of objects was the main selling point for me to use the django-stubs-ext in addition to just django-stubs.

[flaeppe]

I've noted the problem of inheritance with a base that has managers and such too. I've started working out some stuff on it here: #1699

However, noting the type you have on your base's manager; Manager[BasePromotion], I would suggest using ._default_manager attribute where you use your BasePromotion, to get around this for now.

Since ._default_manager aligns perfectly with Manager[BasePromotion] (as long as Manager here is django.db.models.manager.Manager):

django-stubs/django-stubs/db/models/base.pyi

Lines 21 to 23 in dd60767

	class ModelBase(type):
	@property
	def _default_manager(cls: type[_Self]) -> BaseManager[_Self]: ... # type: ignore[misc]

Also note that Manager has nothing in addition to BaseManager. You can verify it by looking at the source:

https://github.com/django/django/blob/814e7bc22062eeae4be9f189e89027e28d5dd290/django/db/models/manager.py#L176-L177

I think this should allow you to avoid having to declare the objects: ClassVar[...] for every subclass of your base

[flaeppe]

To clarify I meant to remove any objects: Manager[BasePromotion] from your BasePromotion model and then use ._default_manager instead of objects whenever the type is type[BasePromotion].

[orsinium]

Thank you, I'll give it a try on Monday and report back here.

[orsinium]

I think now I get what you mean. So, I solved the problem by removing the objects declaration from the base model and copy-pasting it into every submodel.

Before:

class BasePromotion(models.Model):
    objects = PromotionQuerySet.as_manager()

class Promotion(BasePromotion):
    pass

class SubscriptionPromotion(BasePromotion):
    objects = SubscriptionPromotionQuerySet.as_manager()

After:

class BasePromotion(models.Model):
    pass

class Promotion(BasePromotion):
    objects = PromotionQuerySet.as_manager()

class SubscriptionPromotion(BasePromotion):
    objects = SubscriptionPromotionQuerySet.as_manager()

It's not perfect, especially if there are lots of submodels, but should do for now.