title: Symfony2 form type entity expanded extra option
body: <p>I have the following entity field in my form :</p>
  
  <pre><code>    -&gt;add('cover_recto_color', 'entity', [
          'label' =&gt; 'printers_products.cover_recto_color.label',
          'multiple' =&gt; true,
          'expanded' =&gt; true,
          'class' =&gt; 'VeoprintVeoProductBundle:VeoColor',
          'property' =&gt; 'name',
          'query_builder' =&gt; function (EntityRepository $er) {
              return $er-&gt;createQueryBuilder('vc')
                  -&gt;innerJoin('vc.rectoProducts', 'p')
                  -&gt;where('p.family = :family')
                  -&gt;setParameter('family', $this-&gt;_family)
                  ;
          }
      ])
  </code></pre>
  
  <p>This field retrieve a set of data to display it using a list of checkbox. I would like to add another custom choice in this list, representing a NULL value in database but representing a concrete value for the application.</p>
  
  <p>I tried to edit the choices in the <code>finishView</code> function :</p>
  
  <pre><code>public function finishView(FormView $view, FormInterface $form, array $options)
  {
      $new_choice = new ChoiceView(null, 'value', 'label');
      $view-&gt;children['cover_recto_color']-&gt;vars['choices'][] = $new_choice;
  }
  </code></pre>
  
  <p>But it doesn't work. Same for the <code>buildView</code> function.</p>
  
  <p>How can I do?</p>