DEFAULT_AUTO_FIELD feature leads `AbstractModel.check()` to fail with AttributeError

[amureki]

Here is a simple code that shows different behaviour between 3.1 and 3.2:

from django.db import models
class MyAbstractModel(models.Model):
    class Meta:
        abstract = True

# behaviour in 3.1.10:
MyAbstractModel.check() # returns []

# behaviour in 3.2:
MyAbstractModel.check() # raises AttributeError: 'NoneType' object has no attribute 'auto_created' in `_check_default_pk`

It seems to me (but I might be wrong here) that this was not an intentional change (as the feature is all about default autofield), but rather a bug.
However, feel free to correct me!

Best,
Rust

[Johannes Maron]

It is arguable, whether if the checks need to work on an abstract class. I also noticed that Django will raise an error since 3.2 if you try to instantiate an abstract model.
Though, there is no place where checks are called on abstract models inside an application, this does make unit testing harder.

With that in mind, maybe this is best classified as an optimization not a bug, since no public API is broken, not does this affect the application itself. However, since Django is a developer-friendly framework and comes with an excellent test suite, I believe an improvement would be welcome by many.

[amureki]

Replying to Johannes Maron:

It is arguable, whether if the checks need to work on an abstract class. I also noticed that Django will raise an error since 3.2 if you try to instantiate an abstract model.
Though, there is no place where checks are called on abstract models inside an application, this does make unit testing harder.

With that in mind, maybe this is best classified as an optimization not a bug, since no public API is broken, not does this affect the application itself. However, since Django is a developer-friendly framework and comes with an excellent test suite, I believe an improvement would be welcome by many.

Thanks, Johannes!

I made a draft ​PR with blunt solution - freshly introduced auto-field check would not run on abstract models.
I am more than happy to get better ideas or any other feedback!

[Mariusz Felisiak <felisiak.mariusz@…>]

In a24fed39:

Fixed #32733 -- Skipped system check for specifying type of auto-created primary keys on abstract models.

Regression in b5e12d490af3debca8c55ab3c1698189fdedbbdb.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 349bb58b:

[3.2.x] Fixed #32733 -- Skipped system check for specifying type of auto-created primary keys on abstract models.

Regression in b5e12d490af3debca8c55ab3c1698189fdedbbdb.

Backport of a24fed399ced6be2e9dce4cf28db00c3ee21a21c from main