Stop supporting models in non-installed apps

[Aymeric Augustin]

The app registry is explicitly designed to support models outside installed applications.

This doesn't seem particularly useful and makes the code more complicated than necessary.

However, the consequences on backwards compatibility are hairy. If users simply add the missing apps to their INSTALLED_APPS settings, their project could start loading the wrong templates, static files, etc.

This change shouldn't be made lightly.

[Aymeric Augustin]

The 1.7 release notes already warn against this.

[Aymeric Augustin]

This could wreak havoc in Django's own tests, as some create arbitrary model classes.

A good middle ground could be to require that each model:

• is in an installed application, or
• has an explicit app label.

"Being in an installed application" means that the application has been registered in the app registry before the model class gets created. This is related to #21719.

[Aymeric Augustin]

I'm going to implement a deprecation path for this.

[Aymeric Augustin]

PR: ​https://github.com/django/django/pull/2161

[Aymeric Augustin <aymeric.augustin@…>]

In 3326a412ccde9f72e22a070a0b4d922048ed2286:

Deprecated importing a model before loading its application.

Refs #21719, #21680.

[Aymeric Augustin]

This ticket cannot move forward until Django 1.9.

[loic84]

While discussing #19774 on IRC it appeared that django.contrib.sites.models.get_current_site() which is imported in various places in Django would end up being an issue to anyone who doesn't have django.contrib.sites installed.

If we want to be ready for Django 1.9, we probably should start a deprecation period in 1.7 to move get_current_site() to the package __init__.py.

[Aymeric Augustin]

Good point.

[Aymeric Augustin]

PR for contrib.sites: ​https://github.com/django/django/pull/2214

[Carl Meyer]

The PR for contrib.sites looks good.

It occurs to me that the ideal resolution for this ticket would be one where you could safely import models anytime, but they just wouldn't be available in the app registry or register themselves with the ORM (e.g. attach related managers to other models) until/unless their app were installed. (The general principle being that errors on import, or having to be careful what you import when, is an unfortunate smell due to reliance on import side effects.) I presume you considered this and decided it just wasn't feasible to implement? (I haven't looked at the relevant code in detail recently enough to know.)

[Aymeric Augustin]

No, I didn't consider that. I don't know how complicated it would be. In general I would like to see if we can get away with the strict approach and avoid adding complexity to the app registry.

[Aymeric Augustin <aymeric.augustin@…>]

In 9ffab9cee1a5bd1a2f6c326ae970d92526f9a304:

Moved RequestSite and get_current_site.

Following the app-loading refactor, these objects must live outside of
django.contrib.sites.models because they must be available without
importing the django.contrib.sites.models module when
django.contrib.sites isn't installed.

Refs #21680. Thanks Carl and Loic for reporting this issue.

[Aymeric Augustin]

This is still open, but postponed until the deprecation completes.

[Aymeric Augustin]

#22182 was a duplicate (with a long explanation).

[Mark Lavin]

I've created a related ticket which notes that this deprecation exposes an issue with the defaults defined in global settings. See #22477.

[Tim Graham <timograham@…>]

In 4548a282a1c64bb2ed414da3d08617d869cd3740:

Removed contrib.sites.models.RequestSite/get_current_site() aliases.

Per deprecation timeline; refs #21680.

[Tim Graham]

Aymeric will look at this ticket and determine if there is there more to it than promoting ​the existing warning to an exception.

[Tim Graham]

Also need to verify the documentation for Options.app_label in docs/ref/models/options.txt is accurate after this change.

[Aymeric Augustin]

I filed #24312 so as not to lose the idea from comment 12.

[Aymeric Augustin]

Four tests in the test_runner app are failing.

[Aymeric Augustin]

PR: ​https://github.com/django/django/pull/4100

[Aymeric Augustin <aymeric.augustin@…>]

In 1b8af4cfa023161924a45fb572399d2f3071bf5b:

Disallowed importing concrete models without an application.

Removed fragile algorithm to find which application a model belongs to.

Fixed #21680, #21719. Refs #21794.