docs: def save(self, *args, **kwargs):
          """
          ensure users cannot vote the same node multiple times
          but let users change their votes
          """
          if not self.pk:
              old_votes = Vote.objects.filter(user=self.user, node=self.node)
              for old_vote in old_votes:
                  old_vote.delete()
          super(Vote, self).save(*args, **kwargs)