title: Yii: validation rules that always apply except one scenario
body: <p>I know that you can have a validation rule that applies only for one scenario:</p>
  
  <pre><code>array('username', 'exist', 'on' =&gt; 'update'),
  </code></pre>
  
  <p>Now i would like to know if it's possible to do the opposite: a rule that applies everytime except for a given scenrio?</p>
  
  <p>The only solution that is see right now is list all the others scenario, but it's not pretty if we need to add some news scenarios later.</p>
  
  <pre><code>array('username', 'exist', 'on' =&gt; array('create', 'search', ...),//all the scenarios except update
  </code></pre>