[Django]-Aggregate (and other annotated) fields in Django Rest Framework serializers

I made a slight simplification of elnygreen’s answer by annotating the queryset when I defined it. Then I don’t need to override get_queryset().

# views.py
class IceCreamCompanyViewSet(viewsets.ModelViewSet):
    queryset = IceCreamCompany.objects.annotate(
            total_trucks=Count('trucks'),
            total_capacity=Sum('trucks__capacity'))
    serializer_class = IceCreamCompanySerializer

# serializers.py
class IceCreamCompanySerializer(serializers.ModelSerializer):
    total_trucks = serializers.IntegerField()
    total_capacity = serializers.IntegerField()

    class Meta:
        model = IceCreamCompany
        fields = ('name', 'total_trucks', 'total_capacity')

As elnygreen said, the fields must be declared as the serializer’s class attributes to avoid an error about them not existing in the IceCreamCompany model.

You can hack the ModelSerializer constructor to modify the queryset it’s passed by a view or viewset.

class IceCreamCompanySerializer(serializers.ModelSerializer):
    total_trucks = serializers.IntegerField(readonly=True)
    total_capacity = serializers.IntegerField(readonly=True)

    class Meta:
        model = IceCreamCompany
        fields = ('name', 'total_trucks', 'total_capacity')

    def __new__(cls, *args, **kwargs):
        if args and isinstance(args[0], QuerySet):
              queryset = cls._build_queryset(args[0])
              args = (queryset, ) + args[1:]
        return super().__new__(cls, *args, **kwargs)

    @classmethod
    def _build_queryset(cls, queryset):
         # modify the queryset here
         return queryset.annotate(
             total_trucks=...,
             total_capacity=...,
         )

There is no significance in the name _build_queryset (it’s not overriding anything), it just allows us to keep the bloat out of the constructor.

Be careful though when combining multiple aggregations (here trucks and capacity) as it will yield the wrong results because joins are used instead of sub queries.

See the django docs about that:
combining multiple aggregations

In the doc, they suggest to use distinct = True.
For example:

class IceCreamCompanyViewSet(viewsets.ModelViewSet):
queryset = IceCreamCompany.objects.all()
serializer_class = IceCreamCompanySerializer

def get_queryset(self):
    return IceCreamCompany.objects.annotate(
        total_trucks = Count('trucks', distinct=True),
        total_capacity = Sum('trucks__capacity', distinct=True)
    )