title: Django Python Serializer Prefetch
body: <p>I am using the <a href="https://github.com/django/django/blob/stable/1.8.x/django/core/serializers/python.py" rel="nofollow">Django Python Serializer</a> to serialize a list of models which contain a many-to-many relationship. Even with prefetch_related, the serialization retrieves the prefetched-fields. For example:</p>
  
  <pre><code>class House(models.Model):
      name = models.CharField(...)
      rooms = models.ManyToManyField(Door)
  
  class Room(models.Model):
      name = models.CharField(...)
      num_windows = models.PositiveIntegerField(...)
  </code></pre>
  
  <p>Using debug mode I can see that the following function makes the expected 2 database requests.</p>
  
  <pre><code>getHouses():
      House.objects.all().prefetch_related('rooms')
  </code></pre>
  
  <p>However, when I attempt to serialize this object using the django.python.Serializer, it makes an additional query for the rooms in each house. Is there a way to configure the serializer to see the prefetched m2m relationships?</p>