Added a test from kcarnold to show that #7498 is fixed. Refs #7498.

>>> Category.objects.filter(record__left_set__right__category__name='Second').order_by('name')

[<Category: First>, <Category: Second>]

>>> c2 = Child.objects.create(name="Grandchild", parent=c)

Traceback (most recent call last):

    ...

ValueError: Cannot assign "<Child: Child object>": "Child.parent" must be a "Parent" instance.

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