title: Django Rest Framework return nested object using PrimaryKeyRelatedField
body: <p>I am using DRF to expose some API endpoints.</p>
  
  <pre><code># models.py
  
  class Project(models.Model):
      ...
      assigned_to = models.ManyToManyField(
          User, default=None, blank=True, null=True
      )
  
  
  
  # serializers.py
  
  class ProjectSerializer(serializers.ModelSerializer):
      assigned_to = serializers.PrimaryKeyRelatedField(
          queryset=User.objects.all(), required=False, many=True)
  
      class Meta:
          model = Project
          fields = ('id', 'title', 'created_by', 'assigned_to')
  
  
  # view.py
  
  class ProjectList(generics.ListCreateAPIView):
      mode   = Project
      serializer_class = ProjectSerializer
      filter_fields = ('title',)
  
      def post(self, request, format=None):
          # get a list of user.id of assigned_to users
          assigned_to = [x.get('id') for x in request.DATA.get('assigned_to')]
          # create a new project serilaizer
          serializer = ProjectSerializer(data={
              "title": request.DATA.get('title'),
              "created_by": request.user.pk,
              "assigned_to": assigned_to,
          })
          if serializer.is_valid():
              serializer.save()
          else:
              return Response(serializer.errors,
                          status=status.HTTP_400_BAD_REQUEST)
          return Response(serializer.data, status=status.HTTP_201_CREATED)
  </code></pre>
  
  <p>This all works fine, and I can POST a list of ids for the assigned to field. However, to make this function I had to use <code>PrimaryKeyRelatedField</code> instead of <code>RelatedField</code>. This means that when I do a <code>GET</code> then I only receive the primary keys of the user in the <code>assigned_to</code> field. Is there some way to maintain the current behavior for <code>POST</code> but return the serialized <code>User</code> details for the <code>assigned_to</code> field?</p>