title: Using DISTINCT with ActiveRecord
body: <p>I'm using  ActiveRecord / RoR.  </p>
  
  <p>My table looks like this:</p>
  
  <pre><code>id (int), start_time(time), end_time(time), ...
  
  1, 20:00, 23:00
  
  2, 20:00, 23:00
  
  3, 16:00, 20:00
  
  4, 16:00, 23:00
  
  5, 20:00, 22:00
  
  6, 16:00, 20:00
  </code></pre>
  
  <p>I need to return the records that have a combination distinct <code>start_time</code> + <code>end_time</code> combination.</p>