序列化程序中相关对象的有效计数：Django Rest框架

我有一个具有递归关系的模型：

class Update(models.Model):
    created = models.DateTimeField(auto_now_add=True)
    creator = models.ForeignKey(settings.AUTH_USER_MODEL)
    update = models.CharField(max_length=140, null=True, blank=True)
    featured = models.BooleanField(default=False)
    response = models.ForeignKey('self', blank=True, null=True, on_delete=models.SET_NULL, related_name='responding_update')

我正在尝试序列化它，以便它还返回对特定更新的响应数

class UpdateSerializer(serializers.ModelSerializer):
    replies = ReplyCountSerializer(read_only=True)

    class Meta:
        model = Update

class ReplyCountSerializer(serializers.RelatedField):

    def to_representation(self, instance):
        count = ??? Number of responses to the Update ???
        return count

实现我所追求的目标最便宜的方法是什么