Making Session model and SessionStore classes more easily extendable

[Sergey Kolosov]

While developing a feature the involves adding an extra database field to the Session model (thus creating a new session backend based on Django's one), I faced the need to copy almost everything from models.py, backends/db.py and backends/cached_db.py.

The reason why one have to do that, is the way that Session model is imported and used inside backends/db.py and backends/cached_db.py, and the way SessionStore class imported and used inside models.py.

Since the custom backend is a part of a shared package, which is intended to be used with multiple versions of Django, it's challenging to do that given the above.

I suggest the following changes:

• extract an abstract base model from Session;
• make Session class a property of SessionStore class, removing hardcoded usages;
• make key_prefix a property of SessionStore, defaulting to KEY_PREFIX;
• (anything else to make building a custom backend based on Django's one easier).

[Russell Keith-Magee]

Sounds good to me - the sessions framework is designed to be extensible through backends, so we should be doing whatever we can to maximise reuse.

[Sergey Kolosov]

Pull request: ​​https://github.com/django/django/pull/2667

[Sergey Kolosov]

PR updated (documentation and release notes).

[Sergey Kolosov]

PR updated (compatibility with the latest master).

[Sergey Kolosov]

An new (updated) PR: ​https://github.com/django/django/pull/4754

This one also addresses recent Django changes in models/migrations, namely not being able to import models.py of an application which is not in INSTALLED_APPS.

[Tim Graham]

Left some comments for improvement on the PR.

[Sergey Kolosov]

PR updated: ​https://github.com/django/django/pull/4754

[Tim Graham <timograham@…>]

In 22bb5489:

Fixed #22634 -- Made the database-backed session backends more extensible.

Introduced an AbstractBaseSession model and hooks providing the option
of overriding the model class used by the session store and the session
store class used by the model.

[Tim Graham <timograham@…>]

In cc0d1eaa:

Refs #22634 -- Removed unneeded app_label in custom session engine example.

[Tim Graham <timograham@…>]

In 23e106bb:

[1.10.x] Refs #22634 -- Removed unneeded app_label in custom session engine example.

Backport of cc0d1eaaea40dc9d784c6974be1ce631a2087c11 from master

[Tim Graham <timograham@…>]

In 052f849:

[1.9.x] Refs #22634 -- Removed unneeded app_label in custom session engine example.

Backport of cc0d1eaaea40dc9d784c6974be1ce631a2087c11 from master