Is it possible to order order_by with the called?

Question

DUPLICATE: Using the Django Custom Model Method Property in order_by ()

I have two models; which stores messages and another that stores votes made on these posts related to using the ForeignKey field. Each vote is stored as a separate record, since I need to track the user and the date and time when the vote was cast.

I created a helper function that counts all votes using the summation function of Django 1.1.

class Post(models.Model):
    ...some fields...

    def tally(self):
        return self.vote_set.all().aggregate(Sum('value'))['value__sum'] or 0

class Vote(models.Model):
    post = models.ForeignKey(Post)
    value = models.IntegerField()
    ...some fields...

The one query I need to do is to make one value order_byfor the count. But:

Post.objects.all().order_by('tally')

produces the following template error:

: 'tally' . : date_created, , id, is_active, , , slug, user, vote

order_by() ?

Answer 1

, Annotate :

Post.objects.all().annotate(vote_tally=Sum('vote__value')).order_by('-vote_tally')