title: rails 3 validation on uniqueness on multiple attributes
body: <p>I use Rails 3.0.0.beta4</p>
  
  <p>I want to add a validation on uniqueness on two attributes, that means that my model is valid if the couple of <code>'recorded_at'</code> and <code>'zipcode'</code> is unique.</p>
  
  <p>On one attribute here is the syntax</p>
  
  <pre><code>validates :zipcode, :uniqueness =&gt; true
  </code></pre>
  
  <p>thanks</p>