Duplicate exactly identical `ForeignKey`, `ManyToMany`, `OneToOne` fields on child classes raise `AppRegistryNotReady` error

[derthadams]

The Limitations section of the documentation says "Fields defined on subclasses can only be defined on one subclass, unless the duplicate fields are exactly identical".

In testapp/models.py there's an example of two child classes, Developer and Manager, which inherit from the base class Employee and have an exactly identical CharField, along with the comment Adds the _exact_ same field as Developer. Shouldn't error.

I've made a test project using Django 3.2.12 to try out django-typed-models, and I'm finding that exactly identical ForeignKey, ManyToMany, and OneToOne fields on child classes throw an AppRegistryNotReady error, whereas I was expecting them to be de-duplicated like the CharField example.

Here's my models.py:

from django.db import models
from typedmodels.models import TypedModel


class Related(models.Model):
    name = models.CharField(max_length=128)


class Base(TypedModel):
    name = models.CharField(max_length=128)


class ChildA(Base):
    child_a_field = models.CharField(max_length=128, null=True)
    related = models.ForeignKey(
        Related,
        on_delete=models.CASCADE,
        null=True
    )


class ChildB(Base):
    child_b_field = models.CharField(max_length=128, null=True)
    related = models.ForeignKey(
        Related,
        on_delete=models.CASCADE,
        null=True
    )


class ChildC(Base):
    child_c_field = models.CharField(max_length=128, null=True)

When I run python manage.py makemigrations I get:

Traceback (most recent call last):
  File "manage.py", line 22, in <module>
    main()
  File "manage.py", line 18, in main
    execute_from_command_line(sys.argv)
  File "/Users/derth/PycharmProjects/test_dtm/venv/lib/python3.8/site-packages/django/core/management/__init__.py", line 419, in execute_from_command_line
    utility.execute()
  File "/Users/derth/PycharmProjects/test_dtm/venv/lib/python3.8/site-packages/django/core/management/__init__.py", line 395, in execute
    django.setup()
  File "/Users/derth/PycharmProjects/test_dtm/venv/lib/python3.8/site-packages/django/__init__.py", line 24, in setup
    apps.populate(settings.INSTALLED_APPS)
  File "/Users/derth/PycharmProjects/test_dtm/venv/lib/python3.8/site-packages/django/apps/registry.py", line 114, in populate
    app_config.import_models()
  File "/Users/derth/PycharmProjects/test_dtm/venv/lib/python3.8/site-packages/django/apps/config.py", line 301, in import_models
    self.models_module = import_module(models_module_name)
  File "/Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1014, in _gcd_import
  File "<frozen importlib._bootstrap>", line 991, in _find_and_load
  File "<frozen importlib._bootstrap>", line 975, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 671, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 848, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/Users/derth/PycharmProjects/test_dtm/test_dtm/test_fk/models.py", line 22, in <module>
    class ChildB(Base):
  File "/Users/derth/PycharmProjects/test_dtm/venv/lib/python3.8/site-packages/typedmodels/models.py", line 130, in __new__
    if duplicate_field.deconstruct()[1:] != field.deconstruct()[1:]:
  File "/Users/derth/PycharmProjects/test_dtm/venv/lib/python3.8/site-packages/django/db/models/fields/related.py", line 875, in deconstruct
    name, path, args, kwargs = super().deconstruct()
  File "/Users/derth/PycharmProjects/test_dtm/venv/lib/python3.8/site-packages/django/db/models/fields/related.py", line 594, in deconstruct
    swappable_setting = self.swappable_setting
  File "/Users/derth/PycharmProjects/test_dtm/venv/lib/python3.8/site-packages/django/db/models/fields/related.py", line 374, in swappable_setting
    return apps.get_swappable_settings_name(to_string)
  File "/Users/derth/PycharmProjects/test_dtm/venv/lib/python3.8/site-packages/django/apps/registry.py", line 289, in get_swappable_settings_name
    for model in self.get_models(include_swapped=True):
  File "/Users/derth/PycharmProjects/test_dtm/venv/lib/python3.8/site-packages/django/apps/registry.py", line 179, in get_models
    self.check_models_ready()
  File "/Users/derth/PycharmProjects/test_dtm/venv/lib/python3.8/site-packages/django/apps/registry.py", line 141, in check_models_ready
    raise AppRegistryNotReady("Models aren't loaded yet.")
django.core.exceptions.AppRegistryNotReady: Models aren't loaded yet.

The same stack trace also happens with a ManyToManyField or OneToOneField.

As a workaround, I tried deleting the related field from the ChildB model. This allowed me to instantiate ChildB objects with a related field like so:

>>> related = Related.objects.create(name="Related object")
>>> child_b = ChildB.objects.create(name="Child B", child_b_field="My child b field", related=related)

The related field is then accessible (along with all the other columns in the Base table) if you query Base objects:

>>> Base.objects.all().values()
<QuerySet [{'id': 1, 'type': 'test_fk.childb', 'name': 'Child B', 'child_a_field': None, 'related_id': 1, 'child_b_field': 'My child b field', 'child_c_field': None}]>

But it's not accessible if you query ChildB objects:

>>> ChildB.objects.all().values()
<QuerySet [{'id': 1, 'type': 'test_fk.childb', 'name': 'Child B', 'child_b_field': 'My child b field'}]>

It would be great to be able to have the same ForeignKey, ManyToMany or OneToOne field on multiple (but not all) children of a base class.

[danmanix]

Second this exact issue - went straight ahead and defined a couple of subclasses with the same field (copy and paste), and got the same error.

Have moved it to the base class for now, but it's not ideal for admin purposes or what's expected from the docs.

Django 4.1, Python 3.11

Not the end of the world, but an apparent bug?