title: Symfony 2.0: How to validate embedded collection of forms without an entity
body: <p>How can I validate embedded collection of forms (without entities) in Symfony 2.0? Upgrading to 2.1 isn't a convenient option at the moment if the solution lies along that line.</p>
  
  <p>I've tried using the <code>Valid</code> constraint and it caused all validations to fail instead.</p>
  
  <pre><code>public function getDefaultOptions(array $options)
  {
      $collectionConstraint = new Collection(array (
          ...
  
          // I tried Valid constraint but this "removes" all validation
          'travel_links' =&gt; new Valid(),
      ));
  
      return array ('validation_constraint' =&gt; $collectionConstraint);
  }
  </code></pre>