title: Symfony2: string values from choice fields not bound to form object
body: <p>My app contains a form with three choice fields / dropdown lists.</p>
  
  <p>The first is linked to a "relational" field in the entity, and works fine.
  The user is supposed to choose a table link first.</p>
  
  <p>The second and third are both linked to string fields in that same entity.
  Through jQuery, these choice fields get populated with values based on the decision the user makes in the first dropdown list.</p>
  
  <p>After submitting the form, there is an error for the second and third field: they contain invalid input. When I check the form object, their values were not bound; their values never arrived.</p>
  
  <p>The choice fields for collecting string data from the user looks like this:</p>
  
  <pre><code>-&gt;add('sourceName', 'choice', array
      (
          'label' =&gt; 'Choose source name:',
          'empty_value' =&gt; 'Please choose a table link first...',
          'choices' =&gt; array(),
          'attr' =&gt; array('class' =&gt; 'extFieldChoice'),
      )
  </code></pre>
  
  <p>After jQuery has done its job, the html select element looks like this:</p>
  
  <pre><code>&lt;select id="someId" name="someName[sourceName]" required="required" 
    class="extFieldChoice"&gt;
    &lt;option value="first"&gt;first&lt;/option&gt;
    &lt;option value="second"&gt;second&lt;/option&gt;
    &lt;option value="manymore"&gt;Many more...&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>I suspect that the error can be found in the initially empty choices array. However, it would be impossible to fill it with all possible choices, because they run in the hundreds.</p>