title: Symfony2 form choice expended default select
body: <p>I'm doing a filter form in symfony2
  and I try to preselect some filds.</p>
  
  <p>I got two choices:</p>
  
  <pre><code>$formBuilder-&gt;add('first', 'choice',
      array('choices' =&gt; $choiceFirst,
      'expanded' =&gt; false,
       'multiple' =&gt; true,
       'data' =&gt; explode(' ', $this-&gt;getRequest()-&gt;get('first'))
  ));
  $formBuilder-&gt;add('second', 'choice',
      array('choices' =&gt; $choiceSecond,
      'expanded' =&gt; true,
      'multiple' =&gt; true,
      'data' =&gt; explode(' ', $this-&gt;getRequest()-&gt;get('second'))));
  </code></pre>
  
  <p>For the first choice which is not expanded the datas pass by url are preselect.
  But not for the second which is expanded.</p>
  
  <p>There is any ways to preselect checkboxs from a field choice?</p>