Django REST framework: Can a nested object access its parent object's details in a List View?

Question

I am attempting to implement a nested resource where one of its fields depends on a value from its parent resource.

Suppose we are building a system for a company which provides information about its customers plus sales figures for the company's salespeople. So we have two models, Customer and Rep. A rep can sell to more than one customer.

URL that returns all customers: /api/1.0/customers/

URL for a specific customer: /api/1.0/customers/123/

URL for customer-specific information for a specific sales rep : /api/1.0/customers/123/rep/9/

Note the rep URL contains the customer ID as well as the rep ID.

I want the customer URL to return a nested resource containing summary information about the rep, plus a hyperlink to full customer-specific information for that rep. This is the output from the URL for all customers:

[
    {
        "id": 100, 
        "customer_name": "DolManSaxLil",
        "rep": {
                "id": 4,
                "annual_sales": 1500.01,
                "name": "Fred",
                "url": "http://localhost:8000/api/1.0/customer/100/rep/4/"
               }
    },
    {
        "id": 200, 
        "customer_name": "Hotblack",
        "rep": {
                "id": 4,
                "annual_sales": 10500.42,
                "name": "Fred",
                "url": "http://localhost:8000/api/1.0/customer/200/rep/4/"
               }
    }
]

To implement this we define two serializers:

class CustomerSummarySerializer(serializers.HyperlinkedModelSerializer):
    id = ...
    name = ...
    rep = RepSummarySerializer(read_only=True)

class RepSummarySerializer(serializers.HyperlinkedModelSerializer):
    id = ...
    annual_sales = ...
    name = ....
    url = serializers.SerializerMethodField('get_rep_url')

The problem I am facing is that I cannot work out how to access the current customer.id from the function RepSummarySerializer.get_rep_url. It's possible in a Detail view as the customer is held in self.parent.obj:

def get_rep_url(self, obj):
    customer_id = self.parent.obj.id
    url = reverse('api_customer_rep', 
              kwargs={'customer_id': customer_id,
                      'rep_id': obj.id},
                      request=serializer.context.get('request'))
    return url

However, in a list view, self.parent.obj is a QuerySet of Customer objects and I can't see any way of identifying the current Customer. Is there any way of doing this? Have I missed something obvious?

Answer 1

Moment of clarity: the solution is to use a SerializerMethodField to instantiate the RepSummarySerializer and pass the customer_id in the context:

class CustomerSummarySerializer(serializers.HyperlinkedModelSerializer):
    id = ...
    name = ...
    rep = serializers.SerializerMethodField('get_rep')


    def get_rep(self, obj):
        rep = obj.rep
        serializer_context = {'request': self.context.get('request'),
                              'customer_id': obj.id}
        serializer = RepSummarySerializer(rep, context=serializer_context)
        return serializer.data

The customer_id can now be accessed in RepSummarySerializer.get_rep_url like this:

def get_rep_url(self, obj):
    customer_id = self.context.get('customer_id')
    ...

Don't know why I didn't think of this three hours ago.

Answer 2

In addition to the accepted answer, if you use viewsets and want your sub-resource to be a collection (filtered by the parent document) only, you can also use the @detail_route decorator, as documented in the docs:

from rest_framework import viewsets
from rest_framework.decorators import detail_route
from rest_framework.response import Response

class CustomerViewSet(viewsets.ModelViewSet):
    queryset = Customer.objects.all()
    serializer_class = CustomerSummarySerializer

    ...

    @detail_route(methods=['get'])
    def rep(self, request, pk=None):
        customer = self.get_object()
        queryset = customer.pk.all()
        instances = self.filter_queryset(queryset)
        serializer = RepSummarySerializer(instances,
                context={'request': request}, many=True)
        return Response(serializer.data)

Now you can query /customers/123/rep/ and you will get a list of all Rep instances for the specified customer.

It probably won't fully solve what you need, but for many people that don't need full nested resources it's actually enough.