postgresql Django：如何在 post_save 信号中访问原始（未修改）实例
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/5582410/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
Django: How to access original (unmodified) instance in post_save signal
提问by Silver Light
I want to do a data denormalization for better performance, and put a sum of votes my blog post receives inside Post model:
我想对数据进行非规范化以获得更好的性能，并将我的博客文章收到的总投票数放入 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()
Ofcourse, I need to keep Post.ratingvalue actual. Nornally I would use database triggers for that, but now I've decided to make a post_savesignal (to reduce database process time):
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...?
How can I access the instance value before it was saved? In database triggers, i would have OLDand NEWpredefines for this, but is there something like this in post_save signals?
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
更新
The solution based on Mark's the answer:
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()
回答by Mark Lavin
I believe post_saveis too late to retrieve the unmodified version. As the name implies the data has already been written to the db at that point. You should use pre_saveinstead. In that case you can retrieve the model from the db via pk: old = Vote.objects.get(pk=instance.pk)and check for differences in the current instance and the previous instance.
我认为post_save检索未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的差异。
回答by Parag Tyagi -morpheus-
This is not anoptimal solution, but it works.
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
