title: Mongo complex sorting?
body: <p>I know how to sort queries in MongoDB by multiple fields, e.g., <code>db.coll.find().sort({a:1,b:-1})</code>.</p>
  
  <p>Can I sort with a user-defined function; e.g., supposing a and b are integers, by the difference between a and b <code>(a-b)</code>?</p>
  
  <p>Thanks!</p>