Django信号不工作

我有一个自定义函数,当信用添加到CreditsHistory表时运行,该表由信号调用:

def update_credits(sender, instance=None, **kwargs):
    if instance is None:
        return
    sum = CreditsHistory.objects.filter(member=instance.member).aggregate(Sum('credits_amount'))
    instance.member.n_credits = sum['credits_amount__sum']
    instance.member.save()

post_save.connect(update_credits, sender=CreditsHistory)
post_delete.connect(update_credits, sender=CreditsHistory)

某些成员的信用每天都会更新,但对于获得新的CreditHistory实例的所有成员,不会更新成员模型中的n_信用字段。我想可能发生了某种超时,或者数据库无法处理多个成员的大型CreditsHistory表的聚合。。我如何调试它?

编辑:

class CreditsHistory(models.Model):
    member = models.ForeignKey(Member)
    credits_amount = models.IntegerField()
    date = models.DateTimeField(auto_now=True, editable=False)
    notes = models.TextField(blank=True, null=True)

    class Meta:
        ordering = ['-date']
        verbose_name_plural  = "Credit History"

    def __unicode__(self):
        return  u'credits-history/%s' % self.member.user.username

    @classmethod
    def create(cls, member, credits_amount, date, notes):
        creditshistory = cls(member=member, credits_amount=credits_amount, date=date, notes=notes)
        return creditshistory