I want to do a data denormalization for better performance, and put a sum of votes my blog post receives inside Post model:
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. Nornally I would use database triggers for that, but now I've decided to make a
post_save signal (to reduce database process time):
# 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
NEW predefines for this, but is there something like this in post_save signals?
The solution based on Mark's the answer:
# 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()
post_save is 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_save instead. 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.
This is not an optimal solution, but it works.
@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