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

Question

I am trying to figure out the best way to add annotated fields, such as any aggregated (calculated) fields to DRF (Model)Serializers. My use case is simply a situation where an endpoint returns fields that are NOT stored in a database but calculated from a database.

Let's look at the following example:

models.py

class IceCreamCompany(models.Model):     name = models.CharField(primary_key = True, max_length = 255)  class IceCreamTruck(models.Model):     company = models.ForeignKey('IceCreamCompany', related_name='trucks')     capacity = models.IntegerField() 

serializers.py

class IceCreamCompanySerializer(serializers.ModelSerializer):     class Meta:         model = IceCreamCompany 

desired JSON output:

[      {         "name": "Pete's Ice Cream",         "total_trucks": 20,         "total_capacity": 4000     },     ... ] 

I have a couple solutions that work, but each have some issues.

Option 1: add getters to model and use SerializerMethodFields

models.py

class IceCreamCompany(models.Model):     name = models.CharField(primary_key=True, max_length=255)      def get_total_trucks(self):         return self.trucks.count()      def get_total_capacity(self):         return self.trucks.aggregate(Sum('capacity'))['capacity__sum'] 

serializers.py

class IceCreamCompanySerializer(serializers.ModelSerializer):      def get_total_trucks(self, obj):         return obj.get_total_trucks      def get_total_capacity(self, obj):         return obj.get_total_capacity      total_trucks = SerializerMethodField()     total_capacity = SerializerMethodField()      class Meta:         model = IceCreamCompany         fields = ('name', 'total_trucks', 'total_capacity') 

The above code can perhaps be refactored a bit, but it won't change the fact that this option will perform 2 extra SQL queries per IceCreamCompany which is not very efficient.

Option 2: annotate in ViewSet.get_queryset

models.py as originally described.

views.py

class IceCreamCompanyViewSet(viewsets.ModelViewSet):     queryset = IceCreamCompany.objects.all()     serializer_class = IceCreamCompanySerializer      def get_queryset(self):         return IceCreamCompany.objects.annotate(             total_trucks = Count('trucks'),             total_capacity = Sum('trucks__capacity')         ) 

This will get the aggregated fields in a single SQL query but I'm not sure how I would add them to the Serializer as DRF doesn't magically know that I've annotated these fields in the QuerySet. If I add total_trucks and total_capacity to the serializer, it will throw an error about these fields not being present on the Model.

Option 2 can be made work without a serializer by using a View but if the model contains a lot of fields, and only some are required to be in the JSON, it would be a somewhat ugly hack to build the endpoint without a serializer.

Answer 1

Possible solution:

views.py

class IceCreamCompanyViewSet(viewsets.ModelViewSet):     queryset = IceCreamCompany.objects.all()     serializer_class = IceCreamCompanySerializer      def get_queryset(self):         return IceCreamCompany.objects.annotate(             total_trucks=Count('trucks'),             total_capacity=Sum('trucks__capacity')         ) 

serializers.py

class IceCreamCompanySerializer(serializers.ModelSerializer):     total_trucks = serializers.IntegerField()     total_capacity = serializers.IntegerField()      class Meta:         model = IceCreamCompany         fields = ('name', 'total_trucks', 'total_capacity') 

By using Serializer fields I got a small example to work. The fields must be declared as the serializer's class attributes so DRF won't throw an error about them not existing in the IceCreamCompany model.

Answer 2

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.