title: How to serialize a list of objects into list of items for primary key
body: <p>With django-rest-framework, how can I serialize the many elements of a one-to-many relationship into a list of simple values from one field of those items? For example in</p>
  <pre class="lang-py prettyprint-override"><code>class Album(models.Model):
      album_name = models.CharField(max_length=100)
      artist = models.CharField(max_length=100)
  
  class Track(models.Model):
      album = models.ForeignKey(Album, related_name='tracks')
      order = models.IntegerField()
      title = models.CharField(max_length=100)
      duration = models.IntegerField()
  </code></pre>
  <p>I would like to get</p>
  <pre class="lang-json prettyprint-override"><code>{
      'album_name': 'Things We Lost In The Fire',
      'artist': 'Low',
      'tracks': [15, 12, 17, 23]
  }
  </code></pre>