title: ruby convert class name in string to actual class
body: <p>How do I call a class from a string containing that class name inside of it?  (I guess I could do case/when but that seems ugly.)</p>
  
  <p>The reason I ask is because I'm using the <code>acts_as_commentable</code> plugin, among others, and these store the commentable_type as a column.  I want to be able to call whatever particular commentable class to do a <code>find(commentable_id)</code> on it.</p>
  
  <p>Thanks.</p>