Session cached_db backend doesn't use SESSION_CACHE_ALIAS

[CHI Cheng]

cached_db always use default cache backend

[CHI Cheng]

​https://github.com/django/django/pull/1531

[Aymeric Augustin]

Patch looks OK, it would be nice to add tests (or a strong rationale as to why they're impossible to write; I can't remember what support for testing multiple caches we currently have).

This is worth mentioning in the release notes too.

[CHI Cheng]

Added a small test.

A weird thing: the previous code could also pass this test: (copied from CacheSessionTests in contrib/sessions/tests.py)

    @override_settings(CACHES={
        'default': {
            'BACKEND': 'django.core.cache.backends.dummy.DummyCache',
        },
        'sessions': {
            'BACKEND': 'django.core.cache.backends.locmem.LocMemCache',
        },
    }, SESSION_CACHE_ALIAS='sessions')
    def test_non_default_cache(self):
        # Re-initalize the session backend to make use of overridden settings.
        self.session = self.backend()

        self.session.save()
        self.assertEqual(get_cache('default').get(self.session.cache_key), None)
        self.assertNotEqual(get_cache('sessions').get(self.session.cache_key), None)  # Should fail here as previous code always use `default` cache

[Tim Graham <timograham@…>]

In ed9cd4fd8b425ab5010b27aefc7ef6e50d55fb54:

Fixed #21000 -- Made cached_db session backend respect SESSION_CACHE_ALIAS