title: Validation rule for a composite unique index (non-primary)
body: <p>I am sure I am not the first who has composite unique keys in tables and who wants to validate them. I do not want to invent the bicycle so I ask here first. I have several tables that have 'id' columns as primary keys and two other columns as unique composite keys. It would be nice to have a validation rule to check that the submitted entry is unique and display a validation error if it is not. In Cakephp it could be done by a custom validation rule. I am pretty sure somebody has created such method already. </p>
  
  <p>Ideally it would be a method in app_model.php that could be used by different models.</p>