Eu quero fazer uma desnormalização de dados para obter melhor desempenho e colocar uma soma de votos que meu blog recebe dentro do modelo Post:

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()

Claro, eu preciso manterPost.rating valor real. Nornally eu usaria gatilhos de banco de dados para isso, mas agora eu decidi fazer umpost_save sinal (para reduzir o tempo de processo do banco de dados):

# 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...?

Como posso acessar o valor da instância antes de ser salvo? Nos gatilhos de banco de dados, eu teriaOLD eNEW predefine isso, mas há algo assim nos sinais post_save?

ATUALIZA

A solução baseada na resposta de Mark:

# 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()