title: Symfony2 entity type check box set preferred choices
body: <p>I have the following form filed in the edit form.</p>
  
  <pre><code>-&gt;add('district', 'entity', array(
                      'class' =&gt; 'AdminBundle:Districts',
                                 'query_builder' =&gt; function(EntityRepository $repository) {
                  return $repository-&gt;createQueryBuilder('c')
                                      -&gt;where('c.status =:status')
                                      -&gt;setparameter('status','1');
              },
                      'property' =&gt; 'districtName',
                      'preferred_choices' =&gt; array($details-&gt;getDistrict()),
                      'multiple' =&gt; TRUE,
                      'expanded' =&gt; TRUE,
                      'required' =&gt; true,
  
                          )
                  )
  </code></pre>
  
  <p>Output of this is checkboxes. I can check more districts here.
  In the edit mode how to set the preferred choices?</p>