docs: def save(self, *args, **kwargs):
          """
          ensure users cannot rate the same node multiple times
          but let users change their rating
          """
          if not self.pk:
              old_ratings = Rating.objects.filter(user=self.user, node=self.node)
              for old_rating in old_ratings:
                  old_rating.delete()
          super(Rating, self).save(*args, **kwargs)