title: Symfony2 choice field "This value is not valid."
body: <p>I have problem with dynamic forms.</p>
  
  <p>I have two selects. First (name: controller) has a static values. The second one (name: driver) has dynamically values based on value of first select. When I'm changing the first select, new values is generating in second. Everything will be okay but the form didn't want to save.
  I've got this error:</p>
  
  <blockquote>
    <p>This value is not valid.</p>
  </blockquote>
  
  <p><strong>My form builder:</strong></p>
  
  <pre><code>public function buildForm(FormBuilderInterface $builder, array $options)
  {
      $builder
          -&gt;add('controller', null, array(
              'attr' =&gt; array(
                  'id' =&gt; 'controller',
                  'class' =&gt; 'form-control'
              )
          ))
          -&gt;add('driver', 'choice', array(
              'required' =&gt;  false,
              'choices' =&gt; array(),
              'attr' =&gt; array(
                  'id' =&gt; 'driver',
                  'class' =&gt; 'form-control'
              )
          ))
          -&gt;add('update', 'submit', array(
              'label' =&gt; 'form.btn.edit',
              'attr' =&gt; array(
                  'class' =&gt; 'btn btn-default'
              )
          ))
      ;
  }
  
  /**
   * @param OptionsResolverInterface $resolver
   */
  public function setDefaultOptions(OptionsResolverInterface $resolver)
  {
      $resolver-&gt;setDefaults(array(   
          'data_class' =&gt; 'My\ControllerBundle\Entity\Event'
      ));
  }
  </code></pre>
  
  <p>Why can't I save it?</p>