Returning None on errors required unpythonic error checking and was
inconsistent with get_app_config.

get_model was a private API until the previous commit, but given that it
was certainly used in third party software, the change is explained in
the release notes.

Applied the same change to get_registered_model, which is a new private
API introduced during the recent refactoring.

Documented them as public APIs.

ContentTypes are only created for installed applications, and I could
make a case for not returning a model that isn't installed any more.
The check for stale ContentTypes in update_contenttypes doesn't use
model_class.

ModelSignal actually needs get_registered_model since the lookup happens
at import time. I took this opportunity to perform a small refactoring.

This removes the gap between the master app registry and ad-hoc app
registries created by the migration framework, specifically in terms
of behavior of the get_model[s] methods.

This commit contains a stealth feature that I'd rather not describe.

Previously, only the URL path was included in the cache key.

Thanks jamey for the suggestion.

Made it use 'AUTOINCREMENT' suffix for PK creation. This way it doeesn't
regress when compared with the 'traditional' DB backend creation
infrastructure.

Refs #10164.

Thanks to Arfrever for the report and Aymeric for the review.

Thanks kz26 for the report and Tim Graham for the excellent
review.

The last component of the dotted path to the application module is
consistently referenced as the application "label". For instance it's
AppConfig.label. appname could be confused with AppConfig.name, which is
the full dotted path.

register_model is called exactly once in the entire Django code base, at the
bottom of ModelBase.__new__:

    new_class._meta.apps.register_model(new_class._meta.app_label, new_class)

ModelBase.__new__ exits prematurely 120 lines earlier (sigh) if a model with
the same name is already registered:

    if new_class._meta.apps.get_registered_model(new_class._meta.app_label, name):
        return

(This isn't the exact code, but it's equivalent.)

apps.register_model and apps.get_registered_model are essentially a setter and
a getter for apps.all_models, and apps.register_model is the only setter. As a
consequence, new_class._meta.apps.all_models cannot change in-between.

Considering that name == new_class.__name__, we can conclude that
register_model(app_label, model) is always called with such arguments that
get_registered_model(app_label, model.__name__) returns None.

Considering that model._meta.model_name == model.__name__.lower(), and looking
at the implementation of register_model and get_registered_model, this proves
that self.all_models[app_label] doesn't contain model._meta.model_name in
register_model, allowing us to simplify the implementation.

Refs #21692 and 3efd1b8b.

Fixed #21692 -- Quote table name when creating it.

The u prefix looks bad on Python 2.

Thanks nedbatchelder and claudep.

Mostly for consistency with MODELS_MODULE_NAME; it's unlikely to change.

Thanks oubiga for the report.

An admin view performed the capitalization in the template, unlike all others.

Refs #10436.

Thanks Aymeric for the suggestion.

Thanks cjerdonek for the suggestion.

It raises ImportError whenever an entry in INSTALLED_APPS points
(directly or indirectly via AppConfig.name) to a non-existing module
and ImproperlyConfigured in all other cases.

Catching ImportError and re-raising ImproperlyConfigured tends to make
circular imports more difficult to diagnose.

Eliminated the app_ prefix that was more confusing than useful.

Previously the _apps/models_loaded flags could remain set to False if
set_installed_apps exited with an exception, which is going to happen as
soon as we add tests for invalid values of INSTALLED_APPS.

Thanks thepapermen for the report and Carl Meyer for the review.