title: symfony2 override model validation
body: <p>i'm using FOS User Bundle and I want to override the validation file FOS/UserBundle/Resources/config/validaiton.xml:</p>
  
  <p></p>
  
  <pre><code>&lt;constraint name="FOS\UserBundle\Validator\Unique"&gt;
      &lt;option name="property"&gt;usernameCanonical&lt;/option&gt;
      &lt;option name="message"&gt;fos_user.username.already_used&lt;/option&gt;
      &lt;option name="groups"&gt;
          &lt;!-- &lt;value&gt;Registration&lt;/value&gt; --&gt;
          &lt;value&gt;Profile&lt;/value&gt;
      &lt;/option&gt;
  &lt;/constraint&gt;
  </code></pre>
  
  <p>username is not in my Registration form (I just set it to hidden), that's the validation should not produce any error...</p>
  
  <p>Maybe there is a better way to remove the username of the form...</p>