title: Django Request Framework 'ManyRelatedField' object has no attribute 'queryset' when modifying queryset in get_fields
body: <p>I get the following error on <code>GET /api/stories/169/</code> in StorySerializer, noted below in a comment:</p>
  
  <pre><code>AttributeError at /api/stories/169/
  'ManyRelatedField' object has no attribute 'queryset'
  </code></pre>
  
  <p>Upon inspection of the object, I discovered that if I change the line from...</p>
  
  <pre><code>fields['feature'].queryset = fields['feature'].queryset.filter(user=user)
  </code></pre>
  
  <p>to</p>
  
  <pre><code>fields['photos'].child_relation.queryset = fields['photos'].child_relation.queryset.filter(user=user)
  </code></pre>
  
  <p>...it seems to work. But this approach is undocumented, and I'm sure isn't the right way to do it.</p>
  
  <p>I have a these models:</p>
  
  <pre><code>class Story(CommonInfo):
      user = models.ForeignKey(User)
      text = models.TextField(max_length=5000,blank=True)
      feature = models.ForeignKey("Feature", blank=True, null=True)
      tags = models.ManyToManyField("Tag")
  
  class Feature(CommonInfo):
      user = models.ForeignKey(User)
      name = models.CharField(max_length=50)
  
  class Photo(CommonInfo):
      user = models.ForeignKey(User)
      image = ImageField(upload_to='photos')
      story = models.ForeignKey("Story", blank=True, null=True, related_name='photos', on_delete=models.SET_NULL)
  </code></pre>
  
  <p>And a <code>StorySerializer</code>:</p>
  
  <pre><code>class StorySerializer(serializers.HyperlinkedModelSerializer):
      user = serializers.CharField(read_only=True) 
      comments = serializers.HyperlinkedRelatedField(read_only=True, view_name='comment-detail', many=True)
  
      def get_fields(self, *args, **kwargs):
          user = self.context['request'].user
          fields = super(StorySerializer, self).get_fields(*args, **kwargs)
  
          ## Restrict the options that the user can pick to the Features
          ## and Photos that they own
          # This line works:
          fields['feature'].queryset = fields['feature'].queryset.filter(user=user)
  
          # This line throws the error:           
          fields['photos'].queryset = fields['photos'].queryset.filter(user=user)
  
          return fields
  
      class Meta:
          model = Story
          fields = ('url', 'user', 'text', 'comments', 'photos', 'feature', 'tags')
  </code></pre>
  
  <p>What am I doing wrong?  I feel like it's something to do with the direction of the <code>ForeignKey</code> relationships.</p>