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 OLD
and NEW
predefines for this, but is there something like this in post_save signals?
UPDATE
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()
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(pk=instance.pk)
and check for differences in the current instance and the previous instance.