title: Dynamic mongo projection - a projection that uses a field in the document to determine the projection
body: <p>Say I have an object like this:</p>
  
  <pre><code>{default: 'x',
   types: {
    x:1,
    y:2,
    z:3
   }
  }
  </code></pre>
  
  <p>Is it possible to select just types.x (ie a projection of <code>{"types.x":1}</code>) without knowing that x is the default beforehand? Making two queries is clearly possible and not what I'm looking for.</p>