title: Ordering by a related classes created_at in Rails
body: <p>I've got a model User with a related class (User has_many Posts, Post belongs_to User).  I want to display Users in a list, with those who have posted most recently at the top.  So basically I want to order the List of Users by the created_at date of their last post.  How is the best way to query this in Rails 3?</p>
  
  <p>Thanks!</p>