Prefetch's to_attr field is not respected for polymorphic models #498
Description
If I prefetch_related with a to_attr field, the results of the prefetch do not seem to get saved to an attribute on the query results if the prefetch is for a polymorphic model.
So, using an example from the Django docs, if one does:
voted_choices = Choice.objects.filter(votes__gt=0)
prefetch = Prefetch('choice_set', queryset=voted_choices, to_attr='voted_choices')
Question.objects.prefetch_related(prefetch).get().voted_choices
...the final line will result in an error if Choice is a polymorphic model. Using not_polymorphic() on the queryset doesn't seem to help.