title: Rails find all with association
body: <p>I have what I think is a very simple problem (famous last words)...</p>
  
  <p>I have a <code>Category</code> model that <code>has_and_belongs_to_many</code> <code>Events</code>. I want to construct a simple and efficient query that finds all categories that have 1 or more events. (using Rails 3)</p>
  
  <p>I'm sure I'm having a dumb moment here - any help appreciated :)</p>