Django：如何在post_save信号中访问原始（未修改的）实例

Question

我想对数据进行非规范化以获得更好的性能，然后将我的博客文章收到的票数放入Post模型中：

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，我需要保持Post.rating实际价值。通常，我会为此使用数据库触发器，但是现在我决定发出post_save信号（以减少数据库处理时间）：

# 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它？在数据库触发器，我会OLD和NEW这个预定义，但有在post_save信号，这样的事情？

更新

该解决方案基于马克的答案：

# 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

相信post_save检索未修改的版本为时已晚。顾名思义，此时数据已被写入数据库。您应该pre_save改用。在这种情况下，您可以通过pk：从数据库检索模型，old = Vote.objects.get(pk=instance.pk)并检查当前实例和先前实例之间的差异。