今天試驗django級聯查詢的時候老是報錯：'QuerySet' object has no attribute 'user_answer_set'。

查了半天發現QuerySet就算是單條記錄，也是沒法進行級聯查詢的，必須要用單個model物件才行。

錯誤：

paper = User_Paper.objects.filter(id = 21)
answers = paper.user_answer_set.all()

paper = User_Paper.objects.get(id = 21)
answers = paper.user_answer_set.all()

#使用者測試資訊      
class User_Paper(models.Model):
    user_id = models.CharField(max_length=64)
    paper = models.ForeignKey(Paper)
    serialno = models.IntegerField()
    def __unicode__(self):
        return u'%s %s' % (self.user_id, self.paper)
 

#使用者測試答案資訊       
class User_Answer(models.Model):
    user_paper = models.ForeignKey(User_Paper)
    question = models.ForeignKey(Question)
    option = models.ForeignKey(Option)
    def __unicode__(self):
        return u'%s %s' % (self.question, self.option)