title: Symfony2 dynamically modified form ignores default value for new child
body: <p>So I want to modify a form basing on submitted data. I'm using that manual as an example: <a href="http://symfony.com/doc/current/cookbook/form/dynamic_form_modification.html#cookbook-form-events-submitted-data" rel="nofollow">http://symfony.com/doc/current/cookbook/form/dynamic_form_modification.html#cookbook-form-events-submitted-data</a></p>
  
  <p>But also I'd like to provide a default value for the newly added input. To do so I set the data option for the newly added input:</p>
  
  <pre><code>$form-&gt;add('position', 'entity', array(
      'class'       =&gt; 'AcmeDemoBundle:Position',
      'empty_value' =&gt; '',
      'choices'     =&gt; $positions,
      'data'        =&gt; $sport-&gt;getDefaultPosition()
  ));
  </code></pre>
  
  <p>Unfortunately, if the input is added when I process POST_SUBMIT event, it's value gets overridden by the submitted  data. And as there is obviously no corresponding data in the request for this input (it was added dynamically), symfony sets it's value to null. So when I display the enhanced form to a user, it has an empty value in the corresponding field.</p>
  
  <p>What is the best way to overcome this? I can change the submitted data in PRE_SUBMIT event, but that doesn't seem the right way.</p>