DisallowedModelAdminLookup on simple filters of FK fields with inheritance child model as target

[Omar BENHAMID]

To reproduce :

1. Have an FK to a model which inherits another in models.py :

class Parent(Model):
     #...
     pass


class Child(Parent):
     #...
     pass

class FooBar(Model):
      child = ForeignKey(Child, on_delete=models.CASCADE)

2. Have a simple list filter on that fk admin.py:

   class FooBarModelAdmin(ModelAdmin):
       model = FooBar
       list_filter = ("child",)
   

3. Go to the admin of FooBar and try to filter by "child" you get :

   django.contrib.admin.exceptions.DisallowedModelAdminLookup: Filtering by child__parent_ptr__exact not allowed
   

Workaround: override lookup_allowed(...) in the FooBarModelAdmin to allow lookup on child__parent_ptr__exact.

I think that the better fix would be to filter by child_exact rather than child__parent_ptr__exactto avoid exposing some "unnecessary" technical detail, but maybe it has side effects, so the default lookup_allowed(...) should at least allow it.

[Sarah Boyce]

Tested on main and 5.1, couldn't replicate the error