title: Complex forms with Symfony - parent and child entities
body: <p>I'm not quite sure if my title is informative enough.  If anyone with editing privileges wants to tweak it after reading this, feel free.</p>
  
  <p>I need to create a form type that can mirror my current parent&lt;->child, 1-to-1 relationship I have with my entities.  Right now, judging from the docs, the only way I can specify the child entity is to use the <code>Entity</code> form type, but that doesn't seem to give me what I want.  I don't want a selection of existing entities I can add to the parent.  I need to be able to fill in the fields of the parent, and then <em>optionally</em> fill in the fields of the child, all in the same form.  Ideally, I'd be able to do something like:</p>
  
  <pre><code>$builder-&gt;add('child', 'entity', array(
      'required' =&gt; false,
      'class' =&gt; 'MyBundle\Entity\Child',
      'type' =&gt; 'MyBundle\Form\Type\ChildType'
      )
  );
  </code></pre>
  
  <p>But, from what I can see, the <code>type</code> option only exists with collections, which, again, is not what I'm dealing with.</p>
  
  <p>Any suggestions?</p>