51

我想进行数据非规范化以获得更好的性能，并将我的博客文章收到的投票总和放入 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，我会对此进行OLD预定NEW义，但是在 post_save 信号中有类似的东西吗？

更新

基于马克的答案的解决方案：

# 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()
4

3 回答 3

73

我认为post_save检索未修改版本为时已晚。顾名思义，此时数据已经写入数据库。你应该pre_save改用。在这种情况下，您可以通过 pk: 从数据库中检索模型，old = Vote.objects.get(pk=instance.pk)并检查当前实例和前一个实例之间的差异。

于 2011-04-07T15:00:29.330 回答
21

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
于 2019-08-29T07:28:34.873 回答
0

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

于 2020-10-24T03:53:35.747 回答