code: def user_has_reviewed(obj, user):
      
      ctype = ContentType.objects.get_for_model(obj)
      try:
          models.Review.objects.get(user=user, content_type=ctype,
                                    object_id=obj.id)
      except models.Review.DoesNotExist:
          return False
      return True
docs: Returns True if the user has already reviewed the object.