id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
22462	Loading issue of models declared in tests.py due to a combination of AdminConfig, System checks, and Model._meta caching.	loic84	nobody	"I just had a very weird problem with my test suite, where calls to `select_related` didn't seem to work and busted every `assertNumQueries` while still returning correct values. This only happened for models defined in `tests.py` which inherited from models from another app's `models.py`.

{{{
app:
    models.py
        class A(models.Model):
            pass
    admin.py
        @admin.register(A)
        class AAdmin(admin.ModelAdmin):
            list_filters = 'id',
app2:
    tests.py
        class B(A):
            pass
}}}

The problem turned out to be that the various caches in `A._meta` were wrong (those used by `get_field_by_name()` and friends). Calls to `select_related('b')` were silently ignored (because that's what `select_related` do for non-existing relations) but calls to `filter(b=something)` would have failed with a `FieldError ` despite `A` having the `A.b` descriptor.

Here is how the cache got to hold the wrong values:

- `Apps.populate()` calls `AdminConfig.ready()`.
- `AdminConfig.ready()` calls `autodiscover()`.
- `autodiscover()` causes `AAdmin` to register, which in turn triggers `ModelAdmin.check()`.
- `ModelAdmin.check()` inspects the model fields and fills up the caches in `_meta` by doing so.
- `app2.tests.B` is loaded but it's already too late."	Bug	closed	Core (Other)	dev	Normal	duplicate			Accepted	0	0	0	0	0	0