Django的：如何在post_save信號

Question

我想要做一個數據的非規範化以提高性能，並把我的博客文章接收Post模型內的票數總和訪問原始（未修改）實例：

class Post(models.Model): 
    """ Blog entry """ 
    author   = models.ForeignKey(User) 
    title   = models.CharField(max_length=255) 
    text   = models.TextField() 
    rating   = models.IntegerField(default=0) # here is the sum of votes! 

class Vote(models.Model): 
    """ Vote for blog entry """ 
    post   = models.ForeignKey(Post) 
    voter   = models.ForeignKey(User) 
    value   = models.IntegerField() 

Ofcourse，我需要保持Post.rating的實際值。 Nornally我會用數據庫觸發器對於這一點，但現在我已經決定做一個post_save信號（減少數據庫的處理時間）：

# vote was saved 
@receiver(post_save, sender=Vote) 
def update_post_votes(sender, instance, created, **kwargs): 
    """ Update post rating """ 
    if created: 
     instance.post.rating += instance.value 
     instance.post.save() 
    else: 
     # if vote was updated, we need to remove the old vote value and add the new one 
     # but how...? 

我如何可訪問對象的值保存它之前？在數據庫觸發器中，我會爲此預定義OLD和NEW，但在post_save信號中是否有這樣的內容？

UPDATE

根據馬克的答案解決辦法：

# vote was saved 
@receiver(pre_save, sender=Vote) 
def update_post_votes_on_save(sender, instance, **kwargs): 
    """ Update post rating """ 
    # if vote is being updated, then we must remove previous value first 
    if instance.id: 
     old_vote = Vote.objects.get(pk=instance.id) 
     instance.post.rating -= old_vote.value 
    # now adding the new vote 
    instance.post.rating += instance.value 
    instance.post.save() 

Answer 1

我相信post_save是來不及檢索未修改的版本。顧名思義，數據已經被寫入數據庫。您應該使用pre_save。在這種情況下，您可以通過pk：old = Vote.objects.get(pk=instance.pk)從db中檢索模型，並檢查當前實例和以前實例的差異。