App 'category' doesn't provide model 'category' when using abstract base classes

Django: 3.0.6

GitHub: https://github.com/Kifsif/pcask

/category/models.py

class Category(CommentMixin,
               TitleMixin,
               SlugMixin,
               models.Model):
    pass

from category.models import Category

/post/models.py

class Post(TitleMixin,
           SlugMixin,
           DescriptionMixin,
           models.Model):
    ...
    category = models.ForeignKey(Category,
                                 on_delete=models.PROTECT,
                                 related_name='blog_posts')

The database has been dropped and recreated. All migrations have been deleted. In settings.py 'post' and 'category' are added.

When making migrations:

(venv) michael@michael:~/PycharmProjects/pcask/pcask$ python manage.py makemigrations
Traceback (most recent call last):
  File "manage.py", line 21, in <module>
    main()
  File "manage.py", line 17, in main
    execute_from_command_line(sys.argv)
  File "/home/michael/PycharmProjects/pcask/venv/lib/python3.6/site-packages/django/core/management/__init__.py", line 401, in execute_from_command_line
    utility.execute()
  File "/home/michael/PycharmProjects/pcask/venv/lib/python3.6/site-packages/django/core/management/__init__.py", line 395, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/michael/PycharmProjects/pcask/venv/lib/python3.6/site-packages/django/core/management/base.py", line 328, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/michael/PycharmProjects/pcask/venv/lib/python3.6/site-packages/django/core/management/base.py", line 369, in execute
    output = self.handle(*args, **options)
  File "/home/michael/PycharmProjects/pcask/venv/lib/python3.6/site-packages/django/core/management/base.py", line 83, in wrapped
    res = handle_func(*args, **kwargs)
  File "/home/michael/PycharmProjects/pcask/venv/lib/python3.6/site-packages/django/core/management/commands/makemigrations.py", line 168, in handle
    migration_name=self.migration_name,
  File "/home/michael/PycharmProjects/pcask/venv/lib/python3.6/site-packages/django/db/migrations/autodetector.py", line 43, in changes
    changes = self._detect_changes(convert_apps, graph)
  File "/home/michael/PycharmProjects/pcask/venv/lib/python3.6/site-packages/django/db/migrations/autodetector.py", line 129, in _detect_changes
    self.new_apps = self.to_state.apps
  File "/home/michael/PycharmProjects/pcask/venv/lib/python3.6/site-packages/django/utils/functional.py", line 48, in __get__
    res = instance.__dict__[self.name] = self.func(instance)
  File "/home/michael/PycharmProjects/pcask/venv/lib/python3.6/site-packages/django/db/migrations/state.py", line 209, in apps
    return StateApps(self.real_apps, self.models)
  File "/home/michael/PycharmProjects/pcask/venv/lib/python3.6/site-packages/django/db/migrations/state.py", line 279, in __init__
    raise ValueError("\n".join(error.msg for error in errors))
ValueError: The field post.Post.category was declared with a lazy reference to 'category.category', but app 'category' doesn't provide model 'category'.

My mixin classes look like:

class CommentMixin():
    comment = models.TextField(default="", blank=True)

    class Meta:
        abstract = True

What is going on here?

Make your mixin classes inherit models.Model, for example:

class CommentMixin(models.Model):
    comment = models.TextField(default="", blank=True)

    class Meta:
        abstract = True

You can then remove models.Model from your child classes, for example:

class Category(CommentMixin,
               TitleMixin,
               SlugMixin,
               ):
    ...