Migrations autodetector breaking with proxy models inheriting from mixin

[Kevin Marsh]

So I've run into an issue with the new automigrations feature in Django 4.0b1 (and 4.0a1) relating to proxy models inheriting from a non-Model class.

For examples given these three models:

from django.db import models

class MyMixin:
        ...

class MyRealModel(MyMixin, models.Model):
        ...

class MyBaseModel(models.Model):
        ...

class MyProxyModel(MyMixin, MyBaseModel):
        class Meta:
                proxy = True

Running migrate/makemigrations will result in MyProxyModel crashing when it tries running _find_concrete_model_from_proxy:

  File "/some/annonymized/path/django/core/management/commands/makemigrations.py", line 172, in handle
    changes = autodetector.changes(
  File "/some/annonymized/path/django/db/migrations/autodetector.py", line 43, in changes
    changes = self._detect_changes(convert_apps, graph)
  File "/some/annonymized/path/django/db/migrations/autodetector.py", line 156, in _detect_changes
    self.to_state.resolve_fields_and_relations()
  File "/some/annonymized/path/django/db/migrations/state.py", line 449, in resolve_fields_and_relations
    concretes, proxies = self._get_concrete_models_mapping_and_proxy_models()
  File "/some/annonymized/path/django/db/migrations/state.py", line 470, in _get_concrete_models_mapping_and_proxy_models
    concrete_models_mapping[model_key] = self._find_concrete_model_from_proxy(
  File "/some/annonymized/path/django/db/migrations/state.py", line 479, in _find_concrete_model_from_proxy
    base_key = make_model_tuple(base)
  File "/some/annonymized/path/django/db/models/utils.py", line 18, in make_model_tuple
    model_tuple = model._meta.app_label, model._meta.model_name
AttributeError: type object 'MyMixin' has no attribute '_meta'

Changing MyMixin to inherit from models.Model prevents this crash but might not always be possible depending on how the user is using that mixin. Or if we've decided that all inheritance for proxy models must be from models.Model then that should be flagged up more explicitly to developers (eg. checking for it in ModelBase.__new__ or something).