Django:如何在post-save sign中访问原始（未修改）实例

Question

为了获得更好的性能，我想对数据进行非规范化处理，并在post模型中放入我的博客文章获得的投票总数：

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值的实际值。通常我会使用数据库触发器，但现在我决定发出一个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？在数据库触发器中，我会有OLD和NEW的预定义，但是在post-save信号中有类似的东西吗？

更新

基于马克答案的解决方案：

# 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

我相信post_save太晚了，无法检索未修改的版本。顾名思义，此时数据已经写入数据库。您应该改用pre_save。在这种情况下，您可以通过pk:old = Vote.objects.get(pk=instance.pk)从数据库中检索模型，并检查当前实例和前一实例中的差异。

Answer 2

这不是一个最优解，但它是有效的。

@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