title: perform math in searchqueries mongodb
body: <p>Is there a way to perform math in the find function as is possible in
  MySQL? I have the following sql query for performing searches based on
  radius in MySQL:</p>
  
  <pre><code>SELECT id,(3959 * acos(cos(radians(37)) * cos(radians(lat)) * cos( radians(lng) -     radians(-122)) + sin(radians(37)) * sin(radians(lat)))) 
  AS distance 
  FROM markers 
  HAVING distance &lt; 25 
  ORDER BY distance 
  LIMIT 0 , 20;
  </code></pre>
  
  <p>And would like to be able to perform something similar in mongodb or node.js. 
  Is this possible?</p>