Adding extra data to Django Rest Framework results

Question

I'm using Django Rest Framework and need to add extra data to a result set. Specifically, where you would usually have:

{
    "count": 45, 
    "next": "http://localhost:8000/foo/bar?page=2", 
    "previous": null, 
    "results": [
        {...}
    ]
}

I would like to add extra counts like so:

{
    "count": 45,
    "10_mi_count": 10,
    "20_mi_count": 30,
    "30_mi_count": 45,
    "next": "http://localhost:8000/foo/bar?page=2", 
    "previous": null, 
    "results": [
        {...}
    ]
}

The extra counts in this example are just how many of the objects have a field distance with a value less than the miles described in the key.

My issue is that I have no idea where the best place to extend and insert this behaviour is.

Ideally I'd like this to work whether or not the results are paginated, making no assumptions.

What I'm really after here is a nod in the right direction (and why that is the right place for it go).

I've checked the docs and can't find anything that describes how to add things like this, but I would be more than happy to be proven wrong on that score.

Answer 1

Since you seem to be using one of the ListViews from the Rest Framework, you could override the list() method in your class and set new values on the resulting data, like this:

    def list(self, request, *args, **kwargs):
        response = super().list(request, args, kwargs)
        # Add data to response.data Example for your object:
        response.data['10_mi_count'] = 10 # Or wherever you get this values from
        response.data['20_mi_count'] = 30
        response.data['30_mi_count'] = 45
        return response

Notice that your class must inherit the ListModelMixin directly or via a GenericView from the Rest Framework API (http://www.django-rest-framework.org/api-guide/generic-views#listmodelmixin). I really don't know if it is the right way to do this, but it is a quick fix.

Hope it helps!

Answer 2

In the end I just created a custom pagination serializer with a field like so:

class DistanceCountField(serializers.Field):
    def to_native(self, value):
        try:
            distance_counts = {
                '1_mile': self._count_lte_miles(value, 1),
                '5_mile': self._count_lte_miles(value, 5),
                '10_mile': self._count_lte_miles(value, 10),
                '20_mile': self._count_lte_miles(value, 20),
            }
        except FieldError:
            distance_counts = None

        return distance_counts

    def _count_lte_miles(self, value, miles):
        meters = miles * 1609.344
        return value.filter(distance__lte=meters).count()


class PaginatedCountSerializer(pagination.PaginationSerializer):
    distance_counts = DistanceCountField(source='paginator.object_list')

    class Meta:
        # Class omitted, just a standard model serializer
        object_serializer_class = MyModelSerializer 

I also added a distance annotation to each object in the queryset for the filtering to work on.

Answer 3

Use SerializerMethodField as mentioned in this solution.

It can be used to add any sort of data to the serialized representation of your object. (REST framework doc)

Example from the documentation:

from django.contrib.auth.models import User
from django.utils.timezone import now
from rest_framework import serializers

class UserSerializer(serializers.ModelSerializer):
    days_since_joined = serializers.SerializerMethodField()

    class Meta:
        model = User

    def get_days_since_joined(self, obj):
        return (now() - obj.date_joined).days