title: Django NestedHyperlinkedModelSerializer not returning Foreign Key Field
body: <p>Parent id or foreign key is not returning from child. I am trying <a href="https://github.com/alanjds/drf-nested-routers" rel="nofollow noreferrer">drf-nested-routers</a> example.</p>
  <p><em><strong>models.py</strong></em></p>
  <pre><code>class Client(models.Model):
      name = models.CharField(max_length=255)
  
  
  class MailDrop(models.Model):
      title = models.CharField(max_length=255)
      client_id = models.ForeignKey(Client, on_delete=models.CASCADE)
  </code></pre>
  <p><strong>serializers.py</strong></p>
  <pre><code>class ClientSerializer(HyperlinkedModelSerializer):
      class Meta:
          model = Client
          fields = ['id', 'name']
  
  
  class MailDropSerializer(NestedHyperlinkedModelSerializer):
      parent_lookup_kwargs = {
          'client_pk': 'client_id',
      }
  
      class Meta:
          model = MailDrop
          fields = ['id', 'title']
  </code></pre>
  <p><strong>views.py</strong></p>
  <pre><code>class ClientViewSet(viewsets.ViewSet):
      serializer_class = ClientSerializer
  
      def list(self, request):
          queryset = Client.objects.filter()
          serializer = ClientSerializer(queryset, many=True)
          return Response(serializer.data)
  
      def retrieve(self, request, pk=None):
          queryset = Client.objects.filter()
          client = get_object_or_404(queryset, pk=pk)
          serializer = ClientSerializer(client)
          return Response(serializer.data)
  
  
  class MailDropViewSet(viewsets.ViewSet):
      serializer_class = MailDropSerializer
  
      def list(self, request, client_pk=None):
          queryset = MailDrop.objects.filter(client_id=client_pk)
          serializer = MailDropSerializer(queryset, many=True)
          return Response(serializer.data)
  
      def retrieve(self, request, pk=None, client_pk=None):
          queryset = MailDrop.objects.filter(pk=pk, client_id=client_pk)
          maildrop = get_object_or_404(queryset, pk=pk)
          serializer = MailDropSerializer(maildrop)
          return Response(serializer.data)
  </code></pre>
  <p>If I add <code>fields = ['id', 'title', 'client_id']</code> in the <code>MailDropSerializer</code>, it throws the following error:</p>
  <pre><code>AssertionError: `NestedHyperlinkedRelatedField` requires the request in the serializer context. Add `con
  text={'request': request}` when instantiating the serializer.
  </code></pre>
  <p>If I add the request in the serializer context, the output is as follows:</p>
  <pre><code>url: http://127.0.0.1:8000/clients/4/maildrops/
  
  
  [
      {
          &quot;id&quot;: 3,
          &quot;title&quot;: &quot;Madison Mosley&quot;,
          &quot;client_id&quot;: &quot;http://127.0.0.1:8000/clients/4/&quot;
      },
      {
          &quot;id&quot;: 4,
          &quot;title&quot;: &quot;Louis Chen&quot;,
          &quot;client_id&quot;: &quot;http://127.0.0.1:8000/clients/4/&quot;
      }
  ]
  </code></pre>
  <p>I added the serializer context as follows:</p>
  <pre><code>def list(self, request, client_pk=None):
          queryset = MailDrop.objects.filter(client_id=client_pk)
          serializer_context = {
              'request': request
          }
          serializer = MailDropSerializer(
              queryset, many=True, context=serializer_context)
          return Response(serializer.data)
  </code></pre>
  <p>I am trying to get something as follows:</p>
  <pre><code>[
      {
          &quot;id&quot;: 3,
          &quot;title&quot;: &quot;Madison Mosley&quot;,
          &quot;client_id&quot;: 4
      },
      {
          &quot;id&quot;: 4,
          &quot;title&quot;: &quot;Louis Chen&quot;,
          &quot;client_id&quot;: 4
      }
  ]
  </code></pre>