[英]Django: How to access original (unmodified) instance in post_save signal
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,并将我的博文收到的投票总数放入 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.rating
value actual.当然,我需要保持
Post.rating
值实际。 Nornally I would use database triggers for that, but now I've decided to make a post_save
signal (to reduce database process time):通常我会为此使用数据库触发器,但现在我决定制作一个
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...?
How can I access the instance value before it was saved?如何在保存之前访问实例值? In database triggers, i would have
OLD
and NEW
predefines for this, but is there something like this in post_save signals?在数据库触发器中,我会为此预定义
OLD
和NEW
,但是 post_save 信号中是否有类似的东西?
UPDATE更新
The solution based on Mark's the answer: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()
I believe post_save
is too late to retrieve the unmodified version.我相信
post_save
来不及检索未修改的版本。 As the name implies the data has already been written to the db at that point.顾名思义,此时数据已经写入数据库。 You should use
pre_save
instead.您应该改用
pre_save
。 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.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)
并检查当前实例和先前实例的差异。
This is not an optimal solution, but it works.这不是最佳解决方案,但它确实有效。
@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
You can use the FieldTracker from django-model-utils: https://django-model-utils.readthedocs.io/en/latest/utilities.html#field-tracker您可以使用 django-model-utils 中的 FieldTracker: https ://django-model-utils.readthedocs.io/en/latest/utilities.html#field-tracker
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