Implement get_user_model() that can be used in models.py

[Marten Kenbeek]

get_user_model() can only be used when the models for the user app have been loaded into the registry. This can be problematic when you need to subclass the user model, and you need the actual model in your own models.py file. If you want to support swappable models, your app must be after the user app in INSTALLED_APPS.

For reference, this use-case can be found in django-cms: ​https://github.com/divio/django-cms/blob/develop/cms/models/permissionmodels.py#L16

Lets implement an alternative to get_user_model() that can be used after the app registry is populated, but before the user model is loaded. This can work by actually importing the user model if it's not in the registry yet:

from importlib import import_module

from django.apps import apps
from django.conf import settings
from django.utils.module_loading import module_has_submodule

def get_user_model_safe():
    """
    Get the user model before it is loaded, but after the app registry is populated.
    """
    app_label, model_name = settings.AUTH_USER_MODEL.split('.')
    app_config = apps.get_app_config(app_label)
    models_module = import_module("%s.%s" % (app_config.name, "models"))
    return getattr(models_module, model_name)

[Tim Graham]

I think this is a duplicate of what's described in the comments of #22872, although the summary and component of that ticket need to be updated as I assume the original intent of updating the 1.7 release notes with the backwards-incompatibility won't be done at this point. I'm okay with closing either ticket as a duplicate.

[Tim Graham]

This is implemented as the last commit in ​PR #6547.

[Tim Graham]

I left comments for improvement on the PR.

[Tim Graham]

Commits from the branch that precede this fix:

• efcb7e1ebf2b852a442d00a31634438359eb4039 - Modified readiness check in AppConfig.get_model(s).
• fd748c42a96213ebc0a18cc661ecb1abaadabcca - Simplified AppConfig.import_models().
• 625cd5bcb315154ed28061fefaf639aff54da7c2 - Added require_ready argument to get_model methods.

[Markus Holtermann]

I rebased Aymeric's PR onto current master and opened a new PR in ​https://github.com/django/django/pull/7550 .

[Markus Holtermann <info@…>]

In cb7bbf97:

Fixed #25966 -- Made get_user_model() work at import time.

This makes it equivalent to: from django.contrib.auth.models import User.

Thanks Aymeric Augustin for the initial patch and Tim Graham for the
review.