title: Why GenericRelation fields does not work in Data Migrations(Django)
body: <p>I want to make data migration in order to add user read post in database. There is such code:</p>
  
  <pre><code>def user_read_posts(apps, schema_editor):
      User = apps.get_model("main", "User")
      Post = apps.get_model("main", "Post")
      Comment = apps.get_model("comments", "Comment")
  
      comments = Comment.objects.all()
      for comment in comments:
          print (comment.content_object.__class__.__name__)
          if isinstance(comment.content_object, Post):
              comment.user.read_posts.add(comment.content_object)
  
  class Migration(migrations.Migration):
  
      dependencies = [
          ('main', '0039_auto_20160314_0906'),
      ]
  
      operations = [
          migrations.RunPython(user_read_posts),
      ]
  </code></pre>
  
  <p>And in line  <code>print (comment.content_object.__class__.__name__)</code> django raise error:
  <code>AttributeError: 'Comment' object has no attribute 'content_object'</code></p>
  
  <p>Comment model:</p>
  
  <pre><code>class GuidaComment(GenericRelationModel):
  
      user = models.ForeignKey(GuidaUser)
      text = models.TextField()
  </code></pre>
  
  <p>So what should I do?</p>
  
  <p>Thanks.</p>