title: Django rest framework. Return multiple models nested
body: <p>I'm trying to create a combined viewset that displays data in a nested format from three django models.</p>
  
  <p>I'm receiving an error when I try to return the viewset.</p>
  
  <p><code>Got AttributeError when attempting to get a value for field `runner` on serializer `CombinedSerializer`.
  The serializer field might be named incorrectly and not match any attribute or key on the `Runner` instance.
  Original exception text was: 'Runner' object has no attribute 'runner'.</code></p>
  
  <p>serializers.py</p>
  
  <pre><code>class CombinedSerializer(serializers.Serializer):
      event = EventSerializer()
      market = MarketSerializer()
      runner = RunnerSerializer()
      class Meta:
          fields = ('event' , 'market', 'runner')
  </code></pre>
  
  <p>views.py</p>
  
  <pre><code>class CombinedViewSet(mixins.ListModelMixin, 
                        viewsets.GenericViewSet, 
                       mixins.RetrieveModelMixin):
  
      queryset = Runner.objects.all()
      serializer_class = CombinedSerializer
  </code></pre>
  
  <p>models.py</p>
  
  <pre><code>class Event(models.Model):
      sport_id = models.CharField(max_length=15)
      event_id = models.BigIntegerField(unique=True)
      event_name = models.CharField(max_length=200)
      start_time = models.DateTimeField(null=True)
      status = models.CharField(max_length=13)
  
  
  class Market(models.Model):
      event = models.ForeignKey(Event, on_delete=models.CASCADE)
      market_id = models.BigIntegerField(unique=True)
      market_name = models.CharField(max_length=35)
      status = models.CharField(max_length=10)
      volume = models.FloatField(null=True)
  
  class Runner(models.Model):
      event = models.ForeignKey(Event, null=True, default=None, on_delete=models.CASCADE)
      market = models.ForeignKey(Market, null=True, default=None, on_delete=models.CASCADE)
      runner_id = models.BigIntegerField(unique=True)
      name = models.CharField(max_length=500)
      back_odds = models.FloatField(null=True)
      lay_odds = models.FloatField(null=True)
  </code></pre>
  
  <p>For some reason when I remove runner from the serializer Event and Market display in my api in nested format. When I add Runner I get the above error. How can I fix this and display all three in my API?</p>