title: Validate Symfony 2 form with array and entity
body: <p>I'm building a form for array data that embeds an entity type like this:</p>
  
  <pre><code>$data = array('message' =&gt; '', 'book' =&gt; new Book());
  $formBld = $this-&gt;createFormBuilder($data);
  $formBld-&gt;add('book', new BookType(), array(
          'label' =&gt; false,
          'constraints' =&gt; new Symfony\Component\Validator\Constraints\Valid()
       ))
       -&gt;add('message', 'textarea')
       -&gt;add('send', 'submit')
  ;
  </code></pre>
  
  <p>The <code>Book</code> entity contains validation constraints, but they are never called. What is missing/wrong?</p>