title: Get serializer field value in api-view
body: <p>Is it possible to get a field value from a serialized object in Django Rest Framework? Something like this:</p>
  
  <pre><code>serializer = PostSerializer(post)
  print(serializer.title)
  # 'foo title'
  </code></pre>
  
  <p>I need this for fields that are added in the serializer, but are not already on the model, like if the logged in user has liked the post.</p>