How to avoid recalculation when sorting by a model's property in Django Rest Framework?

I have a simple social media app, with User, Post, and PostVote models.

class Post(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    user = models.ForeignKey(User, on_delete=models.SET_NULL, null=True)
    
    ...

    @property
    def score(self):
        total = 0
        for vote in PostVote.objects.all():
            if vote.post == self:
                total += vote.vote
        return total
   
    def get_num_upvotes(self):
        return PostVote.objects.filter(post=self, vote=1).count()

    def get_num_downvotes(self):
        return PostVote.objects.filter(post=self, vote=-1).count()


class PostVote(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    user = models.ForeignKey(User, on_delete=models.SET_NULL, null=True)
    post = models.ForeignKey(Post, on_delete=models.SET_NULL, null=True)
    vote = models.IntegerField()

In my views.py, I attempt to calculate the ‘top’ posts, sorted by score. Here’s the view code.

class ListCreatePostAPIView(ListCreateAPIView):
    serializer_class = PostSerializer
    permission_classes = (IsGoodUser | IsAdminUser,)

    def get_queryset(self):
        try:
            queryset = Post.objects.all().exclude(user=None)
            queryset = list(queryset)
            queryset.sort(key=operator.attrgetter("score"), reverse=True)
            return queryset
        except:
            return Post.objects.none()

In serializers, I also once again recalculate and return the score, as I’d like to pass it to my front-end.

class PostSerializer(serializers.ModelSerializer):
    score = serializers.SerializerMethodField()
    num_upvotes = serializers.SerializerMethodField()
    num_downvotes = serializers.SerializerMethodField()

    def get_score(self, obj):
        return obj.get_score()

    def get_num_upvotes(self, obj):
        return obj.get_num_upvotes()

    def get_num_downvotes(self, obj):
        return obj.get_num_downvotes()

    class Meta:
        model = Post
        fields = (
            "score",
            "num_upvotes",
            "num_downvotes",
        )

I know I’m re-calculating the score way too many times, as it’s taking 3-4 seconds to return just 50 fake posts / 1250 fake PostVotes, but how do I actually make this more efficient? I’ve tried prefetch_related, and I can’t figure out how I’d use it if I’m sorting with a property. I’m really lost and would appreciate any help.

>Solution :

You might find some immediate improvement with using a Django aggregation, rather than a pythonic option that loops through all the objects manually. This is what your score function would become:

@property
def score(self):
    return self.postvote_set.aggregate(Sum('vote')).get('vote__sum')

You can further improve this by caching the result or storing it on the model.

Edit – changed Count to Sum