Model 'Child' is inherited from 'Parent', expecting to get only Child results with Parent.objects.filter(child__isnull=False), but does not work.

[Robin]

Example model:

   class Parent(models.Model): 
        name = models.CharField(max_length='20',blank=True) 
    class Child(Parent): 
        pass 

I want to get the result of Child.objects.all() but I do not want to
use Child.
I want to use Parent instead, which I tried to do with:

    Parent.objects.filter(child__isnull=False) # not work, but should

Which doesn't work and gives the result of Product.objects.all() instead.

However If I do the following, it works:

    Parent.objects.exclude(child__isnull=True) # work

If I insist on using:

    Parent.objects.filter(child__isnull=False) # will work if do below

Then it will work if I use the OneToOneField for Child.

    class Child(models.Model): 
        parent = models.OneToOneField(Parent) 

I expect the query to also work with Model Inheritance as well.

Also discussed in ​http://groups.google.com/group/django-users/browse_thread/thread/9e20b70811f386a8

[Aymeric Augustin]

I'm accepting the bug for two reasons:

• Parent.objects.filter(child__isnull=False) and Parent.objects.exclude(child__isnull=True) should always give the same results.
• Model inheritance and OneToOneField should give the same results — at least that's what the docs say:

  The inheritance relationship introduces links between the child model and each of its parents (via an automatically-created OneToOneField).
  

​https://docs.djangoproject.com/en/dev/topics/db/models/#multi-table-inheritance

It sounds like the wrong kind of join is used.

[Nikolay Zakharov]

The problem is in trim_joins() method of the Query class. This method just trims inner join because the condition is "pk of Child is not null", "pk of Child" is actually a foreign key to parent so trimmer reduces join chain and we finally gets "pk of Parent is not null".

We can avoid it by skipping trimming for the isnull lookup. We also can avoid it by not trimming at all. I think that this problem need better recognition and thinkwork, as it is not only about model inheritance. This behaviour is also reproducible if one has ForeignKey(SomeModel, primary_key=True).

[Tim Graham]

Fixed by #15316 / 2e56066a5b93b528fbce37285bac591b44bc6ed7 which is in Django 1.4+.

The test I used to bisect is:

Parent.objects.create()
self.assertEqual(Parent.objects.filter(child__isnull=False).count(), 0)
self.assertEqual(Parent.objects.filter(child__isnull=True).count(), 1)