title: Django rest-framework + polymorphic: serialize a list of URLs
body: <p>The relevant parts of my model:</p>
  
  <pre><code>class Item(PolymorphicModel):
      rating = models.DecimalField(default=0.0, max_digits=5, decimal_places=2)
      picture = models.URLField(max_length=200)
      category = models.ForeignKey('Category', related_name='items')
      # url_list
  
  class Movie(Item):
      title = models.CharField(max_length=200)
      description = models.CharField(max_length=2000)
      ...
  </code></pre>
  
  <p>I would like to have a list of urls in the Item class. This list should be serialized in the movie object serializer.</p>
  
  <p>So far I tried to create a model to represent a URL this way:</p>
  
  <pre><code>class Url(models.Model):
      url = models.URLField(max_length=200)
      item = models.ForeignKey('Item', related_name='pictures')
  </code></pre>
  
  <p>But I get an empty list when a movie is serialized. My Serializers are:</p>
  
  <pre><code>class UrlSerializer(serializers.ModelSerializer):
  
      class Meta:
          model = ScrollerPictureUrl
          fields = ('url',)
  
  
  class MovieSerializer(serializers.ModelSerializer):
      urls = UrlSerializer(many=True)
  
      class Meta:
          model = Movie
          exclude = ('polymorphic_ctype', 'category')
  </code></pre>
  
  <p>I need to know how to serialize the url list for the Movie object so that it's not empty.</p>