When using .defer on a PolymorphicQuerySet with multiple childs that
subclass from the same polymorphic parent model yield an error like:
>>> Base.objects.defer('ModelY___field_y')
Traceback (most recent call last):
...
FieldDoesNotExist: ModelX has no field named 'field_y'
This avoids the following error in django 1.11 tests:
polymorphic.MRODerived: (models.E005) The field 'id' from parent model 'polymorphic.mrobase3' clashes with the field 'id' from parent model 'polymorphic.mrobase1'.
Related to https://code.djangoproject.com/ticket/22442