Admin app has too hard a dependency on sessions app

[Aarni Koskela]

Since #29695 (371ece2f0682e51f2f796854d3e091827a7cea63), released in 2.2, the admin app checks whether the django.contrib.sessions app is in INSTALLED_APPS.

Some projects may have opted to use a replacement session management app such as ​https://github.com/QueraTeam/django-qsessions – the admin app claims to be incompatible with such a configuration, even if it actually means "I'm going to need _some_ session management that works like django.contrib.sessions".

Maybe it would be better to get rid of the app check and do what's being done for various middleware in the checks function anyway, e.g. something like

if not _contains_subclass('django.contrib.sessions.middleware.SessionMiddleware', settings.MIDDLEWARE):
    errors.append(checks.Error(
        "'django.contrib.sessions.middleware.SessionMiddleware' must "
        "be in MIDDLEWARE in order to use the admin application.",
        id='admin.E4XX',
    ))

– this would be out-of-the-box compatible with e.g. Qsessions.

The obvious workaround is to just re-add django.contrib.sessions back into INSTALLED_APPS which kinda works, but has the mild but unfortunate side effect of forcibly enabling the django.contrib.sessions.models.Session model and migrations, (re-)adding a useless django_session table into the database upon migration.

[Carlton Gibson]

System checks are ​designed to be silenced if not appropriate. I'm inclined to think this just such an edge-case at first glance.

[Aarni Koskela]

I'll work on a PR, then. :)

[Aarni Koskela]

​PR

[Aarni Koskela]

Patch is ready and waiting for review.

[Mariusz Felisiak <felisiak.mariusz@…>]

In efeceba5:

Fixed #30312 -- Relaxed admin check from django.contrib.sessions to SessionMiddleware subclasses.

[Mariusz Felisiak <felisiak.mariusz@…>]

In a4095dad:

[2.2.x] Fixed #30312 -- Relaxed admin check from django.contrib.sessions to SessionMiddleware subclasses.

Backport of efeceba589974b95b35b2e25df86498c96315518 from master