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值实际。 Nornally我会使用数据库触发器，但现在我决定发出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...?

如何在保存之前访问实例值？在数据库触发器中，我会为此OLD和NEW预定义，但在post_save信号中是否有这样的东西？

更新

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

Answer 2

这不是不是最佳解决方案，但可以。

@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

Answer 3

您可以使用django-model-utils中的FieldTracker：https://django-model-utils.readthedocs.io/en/latest/utilities.html#field-tracker

Django：如何在post_save信号中访问原始（未修改的）实例

3 个答案: