Annotating a Sum results in None rather than zero

Question

I'm making a QA site that is similar to the page you're on right now. I'm attempting to order answers by their score, but answers which have no votes are having their score set to None rather than 0. This results in answers with no votes being at the bottom of the page below negatively ranked answers. How can I make the annotated score be zero when there are no votes for an answer?

Here's my model:

from django.contrib.auth.models import User

Answer(models.Model):
    //some fields here
    pass

VOTE_CHOICES = ((-1, Down), (1, Up))

Vote(models.Model):
    user = models.ForeignKey(User)
    answer = models.ForeignKey(Answer)
    type = models.IntegerField(choices = VOTE_CHOICES)

    class Meta:
        unique_together = (user, answer)

And here's my query:

answers = Answer.objects.filter(<something here>)
                        .annotate(score=Sum('vote__type'))
                        .order_by('-score')

edit: And to be clear, I'd like to do this in the query. I know I could turn it into a list and then sort it in my python code, but I'd like to avoid that if possible.

Answer 1

You could use this workaround, but I'm not sure how compatible this is to different databases (works in sqlite3, though):

Answer.objects.extra(select={
     'score': 'IFNULL((SELECT SUM(type) ' + \
                       'FROM <yourapp>_vote ' + \
                       'WHERE <yourapp>_vote.answer_id = ' + \
                             '<yourapp>_answer.id), ' + \
                      '0)'
}).order_by('-score')

Answer 2

What about you use custom Manager? For example:

AnswerManager(models.Manager):
    def all_with_score(self):
       qs = self.get_query_set().annotate(score=Sum('vote__type'))
       # Here, you can do stuff with QuerySet, for example
       # iterate over all Answers and set 'score' to zero if None.

Answer(models.Model):
    //some fields here
    objects = AnswerManager()

Then, you can use:

>>> answers = Answer.objects.all_with_score().order_by('-score')

Answer 3

You can use the Coalesce function from django.db.models.functions like:

answers = Answer.objects.filter(<something here>)
                        .annotate(score=Coalesce(Sum('vote__type'), 0))
                        .order_by('-score')