Allow FieldTracker on Django user model (and other children of abstract base classes)

Problem

Here is a branch with failing tests that I think should work (and that I think worked in earlier versions?): https://github.com/jazzband/django-model-utils/compare/master...jcushman:abstract-test-failure

This seems to be related to the stuff that @lucaswiman added in #317. @lucaswiman, I’m hoping based on that work you might have some clever idea for how to fix this. 😃

Here’s the situation: you have an abstract base class that defines a static attribute like is_active = True, and a concrete model inheriting from that class that defines a field like is_active = models.BooleanField(default=True). The model then throws an error on save():

from django.contrib.auth.models import AbstractUser

class MyUser(AbstractUser):
    tracker = FieldTracker()

MyUser().save()

Result:

    def __get__(self, instance, owner):
        if instance is None:
            return self
        was_deferred = self.field_name in instance.get_deferred_fields()
>       value = self.descriptor.__get__(instance, owner)
E       AttributeError: 'bool' object has no attribute '__get__'

model_utils/tracker.py:43: AttributeError

It would be great for this to work, because tracking changes on the Django user model is handy.

Debugging that error, I found the problem boils down to this:

class AbstractUser(models.Model):
    is_active = True

    class Meta:
        abstract = True

class MyUser(AbstractUser):
    is_active = models.BooleanField(default=True)
    tracker = FieldTracker()

MyUser().save()

The reason that fails is https://github.com/jazzband/django-model-utils/blob/master/model_utils/tracker.py#L218 :

descriptor = getattr(sender, field_name)
...
setattr(sender, field_name, wrapped_descriptor)

… which boils down to setting MyUser.is_active = DescriptorWrapper(MyUser.is_active). And that doesn’t work because you expect MyUser.is_active to start with a value of DeferredAttribute('is_active'), but it actually returns True. For reasons I don’t understand, when you override a static attribute on an abstract class, you get the base class’s value back instead of the subclass’s.

I tried tweaking tracker.py with variations on descriptor = getattr(sender, field_name) if field_name in sender.__dict__ else DeferredAttribute(field_name), but that broke foreign key fields and feels pretty janky anyway.

Any ideas on how to get this working?

Thanks!

Environment

• Django Model Utils version: master
• Django version: 2.1
• Python version: 3.6
• Other libraries used, if any:

[dmugtasimov]

The reason for this issue is that django.contrib.auth.models.AbstractBaseUser has class level attribute is_active:

class AbstractBaseUser(models.Model):
    password = models.CharField(_('password'), max_length=128)
    last_login = models.DateTimeField(_('last login'), blank=True, null=True)

    is_active = True

(Pdb++) instance.is_active
True
(Pdb++) instance.is_active = False
(Pdb++) instance.is_active
True

After applying this work around it works just fine:

if hasattr(AbstractBaseUser, 'is_active'):
    del AbstractBaseUser.is_active

(Pdb++) instance.is_active
False
(Pdb++) instance.is_active = True
(Pdb++) instance.is_active
True
(Pdb++) instance.is_active = False
(Pdb++) instance.is_active
False

I think it deserves a fix either in django-model-utlils or in Django.

[mxschmitt]

The issue for me is still persistent in version 3.2.0. 😕 Same behavior with the is_active attribute like described above.