title: cakephp multiple select validation
body: <p>I am using CakePHP and i have something like:</p>
  
  <p>PRODUCT -------> PRODUCT_CATEGORY &lt;---------- CATEGORY</p>
  
  <p>so one product can have 'n' categories and viceversa.
  The problem is that i would like to validate the products so that the have at least one category. Since I am using the Form assistant and the validate functions of CakePHP y have arrived to this:</p>
  
  <pre><code>class Product extends AppModel {
      var $name = 'Product';
      var $validate = array(
          'category_id' =&gt; array(
              'rule' =&gt; array('multiple', array('min' =&gt; 1)),
              'message' =&gt; 'You have to choose at least one category'
          )
      );
  }
  </code></pre>
  
  <p>But it doesn't work, any ideas?</p>