title: Sort by number of subdocuments mongoid
body: <p>I have a model <code>Person</code> that has many <code>Comments</code></p>
  
  <p>How could I sort people collection by number of <code>comments</code></p>
  
  <p>Something like </p>
  
  <blockquote>
    <p>Person.desc(:"comments.count")</p>
  </blockquote>