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:

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.value
        # 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 OLD and 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
        old_vote = Vote.objects.get( -= old_vote.value
    # now adding the new vote += instance.value
4/8/2011 11:22:09 AM

Accepted Answer

I believe 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( and check for differences in the current instance and the previous instance.

4/7/2011 3:00:29 PM

This is not an optimal solution, but it works.

@receiver(pre_save, sender=SomeModel)
def model_pre_save(sender, instance, **kwargs):
        instance._pre_save_instance = SomeModel.objects.get(
    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 

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