title: Django rest framework custom serialization
body: <p>I want to convert raw JSON:</p>
  
  <pre><code>{'code': 1, 'inner': {'x': 1, 'y': 1}}
  </code></pre>
  
  <p>To rendered serializer.data:</p>
  
  <pre><code>{'code': 1, 'inner_x': 1, 'inner_y': 1}
  </code></pre>
  
  <p>with django REST framework serializer.</p>
  
  <p>Which code should be placed in serializer?</p>