title: SonataAdminBundle sonata_type_collection not showing field
body: <p>I am using SonataAdminBundle. I want to add in my creation form ability to add one to many ralation objects. So I am trying to use sonata_type_collection like this:</p>
  
  <pre><code>$formMapper-&gt;add('attachments', 'sonata_type_collection', array(
                      'label' =&gt; 'Attachments',
                      // Prevents the "Delete" option from being displayed
                      'type_options' =&gt; array('delete' =&gt; false)
                  ), array(
                      'edit' =&gt; 'inline',
                      'inline' =&gt; 'table',
                      'sortable' =&gt; 'position',
                  ))
  </code></pre>
  
  <p>But my creation form renders only a label of the field, nothing more. </p>
  
  <p>Thanks for all suggestions in advance :)</p>