title: How to set SlugRelated field to a field within an object field
body: <p>I have the following models:</p>
  
  <pre><code>class Category(models.Model):
      name = models.CharField()
      ... # fields omitted
  
  class Prediction(models.Model):
      conversation = models.ForeignKey(Conversation)
      category = models.ForeignKey(Category)
      ... # fields omitted
  
  class Conversation(models.Model):
      sid = models.CharField()
      ... # fields omitted
  </code></pre>
  
  <p>Now I'm trying to create a model serializer for <code>Category</code> that would return me the following serialized object:</p>
  
  <pre><code>{
      "name":"blah",
      "conversations":[
          "2af22188c5c97256",    # This is the value of the sid field
          "073aef6aad0883f8",
          "5d3dc73fc8cf34be",
       ]
  }
  </code></pre>
  
  <p>Here is what I have in my serializer:</p>
  
  <pre><code>class CategorySerializer(serializers.ModelSerializer):
      conversations = serializers.SlugRelatedField(many=True,
               read_only=True, 
               source="prediction_set", 
               slug_field='conversation.sid')
  
      class Meta:
          model = models.Class
          fields = ('class_name', 'conversations')
  </code></pre>
  
  <p>However, this doesn't work because somehow django doesn't allow me to set slug_field to be a field that's within an object field. Any suggestions on how to accomplish this?</p>