title: Mongoose query which compares field values
body: <p>Say i have the following Scheme</p>
  
  <pre><code>TestSchema{
   val1:Number,
   val2:Number}
  </code></pre>
  
  <p>Would it be possible to write a query where all the results returned are where val1=val2 ?</p>
  
  <p>If so , is there a way to use it when doing a populate ? (Perhaps inside the match field in the opts object ?)</p>