title: hash instead of id
body: <p>I want to use auto-generated hash'es instead of auto-incremented integers in my activerecords as primary keys. This raises two questions:</p>
  
  <ol>
  <li>how to perform this generation in
  most efficient way?</li>
  <li>how to handle possibility that
  generated hash exists already in
  table?</li>
  </ol>
  
  <p>Regards, 
  Mateusz</p>