title: Symfony2 collection and related choice field not (yet) managed
body: <p>I've got a form with a collection field "options" and a choice field "defaultOption". My goal is to enable the user to create a new entity, add some options and select a default option from the list of option he/she just added before saving the entire thing.</p>
  
  <p>Here's part of the form that shows both fields:</p>
  
  <pre><code>            $form-&gt;add('options', 'collection', array(
                  'type' =&gt; new CustomFieldOptionType(),
                  'label' =&gt; 'custom.field.options',
                  'allow_add' =&gt; true,
                  'allow_delete' =&gt; true,
                  'options' =&gt; array(
                      'label_render' =&gt; false,
                      'widget_control_group' =&gt; false,
                  ),
                  'by_reference' =&gt; false,
                  'attr' =&gt; array('class' =&gt; 'options')
              ));
  
              $form-&gt;add('defaultOption', 'entity', array(
                  'label' =&gt; 'custom.field.default',
                  'class' =&gt; 'XFDOBundle:CustomFieldOption',
                  'choices' =&gt; $field-&gt;getOptions(),
                  'property' =&gt; 'name',
                  'required' =&gt; false,
                  'empty_value' =&gt; 'custom.field.default.empty',
                  'attr' =&gt; array('class' =&gt; 'default-option')
              ));
  </code></pre>
  
  <p>I've tried several things with jquery and form event listeners to (re)populate the "defaultOption" field with the new (not-yet-persisted) options, but that resulted in to nothing or "Entities passed to the choice field must be managed" exceptions.</p>
  
  <p>Any idea how I can get this working?</p>