title: Unique on three different fields
body: <p>I am working on a Ruby on Rails web application. I want to validate the uniqueness of more than one field together. How can i do this?</p>
  
  <p>For example: I have a model named <code>waiting</code> with three fields:  </p>
  
  <p><code>project_id</code> <code>category_id</code> and <code>user_id</code>  </p>
  
  <p>I want to ensure that i won't have two identical rows in <strong>all</strong> three fields.</p>