Django queryset 'in' operator on two querysets fails on first call

Cross posted from: stack overflow.

When using the 'in' operator on two querysets, the first time the call is made it fails.

from django.db import models    class Category(models.Model):      name = models.CharField(max_length=100)      slug = models.SlugField(max_length=100, db_index=True, unique=True)      def __unicode__(self):          return self.name    class Director(models.Model):      name = models.CharField(max_length=100)      slug = models.SlugField(max_length=100, db_index=True, unique=True)      def __unicode__(self):          return self.name    class Project(models.Model):      title = models.CharField(max_length=100)      slug = models.SlugField(max_length=100, db_index=True, unique=True)      director = models.ForeignKey(Director, related_name='projects_via_director')      categories = models.ManyToManyField(Category,                                           related_name='projects_via_category')          def __unicode__(self):          return self.title      category_list = Category.objects.prefetch_related('projects_via_category')  director_list = Director.objects.prefetch_related('projects_via_director')  other_cats = category_list.filter(      projects_via_category__director__slug='steven-spielberg')    print category_list # [<Category: Film>, <Category: Commercial>]  print other_cats # [<Category: Film>]  print category_list[0] in other_cats # False  print category_list[0] in other_cats # True

If I remove 'prefetch_related' and use 'all' instead then the problem is gone, however this uses many more DB calls.

How can I get the correct result first time using prefetch_related?

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