title: Symfony2 verify a form checkbox is checked in twig template
body: <p>I have a problem of verify a check box in twig template. In twig, i wanna to do this:</p>
  
  <pre><code>{% for activity in form.activity %}
      {% if (activity.isChecked) %}
      &lt;div&gt;
          {{ form_widget(activity) }}
          {{ form_label(activity) }}
      &lt;/div&gt;
      {% endif  %}
  {% endfor %}
  </code></pre>
  
  <p>The activity is a entity field in form builder:</p>
  
  <pre><code>-&gt;add('activity', 'entity', array(
              'class'         =&gt; 'AcmeProspectionBundle:Activity',        
              'expanded'      =&gt; true,
              'multiple'      =&gt; true,
              'property'      =&gt; 'name',
  /*
              'query_builder' =&gt; function(EntityRepository $er) use($options) {
                  return $er-&gt;createQueryBuilder('ac')
                  -&gt;leftJoin('ac.company','c')
                  -&gt;where('c = :id')
                  -&gt;orderBy('ac.name', 'ASC')
                  -&gt;setParameter('id', $options['company_id']);
              }
  */
  ))
  </code></pre>
  
  <p>In fact, if I do not comment the <em>query_builder</em> part, it will only generate the checked part, but if I do this Symfony will check the integrity of the original array and the submit array. I can not add any new activity. In fact I generate the choice by javascript like this:</p>
  
  <pre><code> &lt;div class="activity_checkbox" data-prototype='&lt;div&gt;
   &lt;input type="checkbox" checked="checked" value="100" placeholder="" name="acme_prospection_company[activity][]"&gt;
   &lt;label&gt;&lt;/label&gt;&lt;/div&gt;'&gt;
  </code></pre>
  
  <p>Cause I have more than 3000 choice and I can not use the original checkbox or select to let user do that. So now all the work is done, only need to render the checked activities. And I do not know how.</p>