title: Django GenericRelation fields not available during South migration
body: <p>In a Django project, I have models defined like this :</p>
  
  <pre><code>from django.db import models
  from django.contrib.contenttypes.models import ContentType
  from django.contrib.contenttypes import generic
  
  class TaggedEntry(models.Model):
      content_type = models.ForeignKey(ContentType)
      object_id = models.PositiveIntegerField()
      content_object = generic.GenericForeignKey("content_type", "object_id")
  
      class Meta:
          abstract = True
  
  class File(TaggedEntry):
      name = models.CharField(max_length = 256)
      # some more fields
  
  class Folder(models.Model):
      name = models.CharField(max_length = 200)
      files = generic.GenericRelation(File)
      # some more fields
  </code></pre>
  
  <p>In the project I can use them this way :</p>
  
  <pre><code>folder = Folder.objects.get(name="fooo")
  for f in folder.files.iterator():
      print f.name
  </code></pre>
  
  <p>I'm now preparing a datamigration with <code>South</code> in which I need to access the files of the folders but the code <code>folder.files.iterator()</code></p>
  
  <p>gives me an error :</p>
  
  <pre><code>Error in migration: main:0015_contenttype_to_manytomany_step0
  AttributeError: 'Folder' object has no attribute 'files'
  </code></pre>
  
  <p>Is it expected?</p>
  
  <p>How can I know the files being part of a folder?</p>