Django：如何访问post_save信号中的原始（未修改）实例

问题描述

我想做一个数据非规范化以获得更好的表现，并且在博客模型中投入一张我的博客文章：

I want to do a data denormalization for better performance, and put a sum of votes my blog post receives inside Post model:

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()

当然，我需要保留 Post.rating value actual。 Nornally我会使用数据库触发器，但现在我已经决定做一个 post_save 信号（以减少数据库处理时间）：

Ofcourse, I need to keep Post.rating value actual. Nornally I would use database triggers for that, but now I've decided to make a post_save signal (to reduce database process time):

# 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...?

如何在保存实例值之前访问实例值？在数据库触发器中，我会为此定义 OLD 和 NEW 预定义，但是在post_save信号中有这样的东西吗？

How can I access the instance value before it was saved? In database triggers, i would have OLD and NEW predefines for this, but is there something like this in post_save signals?

更新

基于Mark的解决方案：

The solution based on Mark's the answer:

# 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()

推荐答案

相信 post_save 太晚无法检索未修改的版本。顾名思义，数据已经被写入db。您应该使用 pre_save 。在这种情况下，您可以通过pk从db获取模型： old = Vote.objects.get（pk = instance.pk）并检查当前实例和前一个例子。

I believe post_save is too late to retrieve the unmodified version. As the name implies the data has already been written to the db at that point. You should use pre_save instead. In that case you can retrieve the model from the db via pk: old = Vote.objects.get(pk=instance.pk) and check for differences in the current instance and the previous instance.

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