title: Mongoid order_by boolean
body: <p>I have a notifications table in my database which contains the attributes: <code>timestamp:datetime</code> and <code>read:boolean</code>. What I want is to query and order my notifications; first by if they are read or not, then after which timestamp they've got, then limit the amount of notifications to 10.</p>
  
  <p>The query I've tried looks something like this:</p>
  
  <pre><code>@user.notifications.order_by([[:read,:desc],[:timestamp,:desc]]).limit(10)
  </code></pre>
  
  <p>Which only gives me an error which I narrowed down to proving the boolean field as the culprit.</p>
  
  <p>Is there an existing way of ordering by true/false values or should I resort to using some kind of Custom Field Serialization transforming True's and False's to 1's and 0's?</p>