Django / DRF: annotate a model with extra (computed) information

Question

I'm adding a very simple roadmap voting feature to my website, where people can add feature requests, and people can then vote on each other's suggestions. The basics are pretty simple:

# models.py
class FeatureRequest(models.Model):
    title = models.CharField(max_length=50)
    description = models.TextField()
    author = models.ForeignKey(User, editable=False, on_delete=models.CASCADE)
    is_implemented = models.BooleanField(default=False, editable=False, db_index=True)
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)


class Vote(models.Model):
    feature = models.ForeignKey(FeatureRequest, editable=False, on_delete=models.CASCADE)
    user = models.ForeignKey(User, editable=False, on_delete=models.CASCADE)
    created_at = models.DateTimeField(auto_now_add=True)

    class Meta:
        unique_together = ["feature", "user"]

#views.py
class RoadmapController(viewsets.ModelViewSet):
    permission_classes = (AuthorOrReadOnlyPermission,)
    serializer_class = FeatureRequestSerializer

    def get_queryset(self):
        return FeatureRequest.objects.filter(is_implemented=False)

    def perform_create(self, serializer):
        return serializer.save(author=self.request.user)

# serializers.py
class FeatureRequestSerializer(serializers.ModelSerializer):
    class Meta:
        model = FeatureRequest
        fields = "__all__"

There is also a separate view for actually creating and deleting votes, but that's not needed here for my question.

What I want is that the response of the feature request list includes the number of votes for each one, and a boolean if the logged-in user has voted for it:

[
  {
    "title": "This is a feature request", 
    "description": "Foo bar", 
    "author": 1, 
    "number_of_votes": 1, 
    "has_my_vote": true
  }
]

I already figured out that I can change my queryset to FeatureRequest.objects.filter(is_implemented=False).annotate(number_of_votes=Count("vote")), add number_of_votes = serializers.ReadOnlyField() to the serializer, and the number of votes is visible. Is that the best way? It adds a query for each feature request I would assume.

But most of all, I don't know how to add that "has_my_vote" boolean to the result (and how to keep that performant, of course).

Answer 1

After some trial and error, I came up with this:

    def get_queryset(self):
        return (
            FeatureRequest.objects.filter(is_implemented=False)
            .annotate(number_of_votes=Count("vote"))
            .annotate(has_my_vote=Exists(Vote.objects.filter(feature=OuterRef("pk"), user=self.request.user)))
        )

That works, and it only does one query instead of one plus one or two per feature request.