Django:如何在post_save信号中访问原始(未修改的)实例

Question

我想做一个数据非规范化以获得更好的性能,并在Post模型中收到我的博客帖子收到的一些投票:

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.Nornally我会使用数据库触发器,但现在我决定发出一个post_save信号(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...?

如何在保存之前访问实例值？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？

UPDATE

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

我认为post_save检索未经修改的版本为时已晚.顾名思义,此时数据已经写入数据库.你应该使用pre_save.在这种情况下,您可以通过pk从db中检索模型,old = Vote.objects.get(pk=instance.pk)并检查当前实例和上一个实例的差异.

Answer 2

这不是最佳解决方案，但它有效。

@receiver(pre_save, sender=SomeModel)
def model_pre_save(sender, instance, **kwargs):
    try:
        instance._pre_save_instance = SomeModel.objects.get(pk=instance.pk)
    except SomeModel.DoesNotExist:
        instance._pre_save_instance = instance


@receiver(signal=post_save, sender=SomeModel)
def model_post_save(sender, instance, created, **kwargs):
    pre_save_instance = instance._pre_save_instance
    post_save_instance = instance