title: YML validation file is ignored
body: <p>I am new to the Symfony 2 web framework, and am struggling with a very basic validation task. I have an entity model <code>Post</code> that has a member <code>slug</code>, which I use to build the link to the post. In <code>Post.orm.yml</code> I defined <code>unique: true</code> and want to include this constraint as a validator as well.</p>
  
  <p>I have created a file <code>validation.yml</code>:</p>
  
  <pre><code># src/OwnBundles/BlogpostBundle/Resources/config/validation.yml
  
  OwnBundles\BlogpostBundle\Entity\Post:
      properties:
          slug:
              - NotBlank: ~
      constraints:
          - Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity: slug
  </code></pre>
  
  <p>The creation function in my controller is quite simple:</p>
  
  <pre><code>public function addAction(Request $request)
  {
      $post = new Post();
      $form = $this-&gt;createForm(new PostType(), $post);
  
      if($request-&gt;getMethod() == 'POST')
      {
          $form-&gt;bind($request);
          if($form-&gt;isValid())
          {
              $em = $this-&gt;getDoctrine()-&gt;getManager();
              $em-&gt;persist($post);
              $em-&gt;flush();
              return $this-&gt;redirect(
                  $this-&gt;generateUrl('own_bundles_blogpost_homepage')
              );
          }
      }
      return $this-&gt;render(
          'OwnBundlesBlogpostBundle:Default:add.html.twig',
          array(
              'title' =&gt; 'Add new blogpost',
              'form' =&gt; $form-&gt;createView(),
          )
      );
  }
  </code></pre>
  
  <p>The basic pageflow works fine, I can add posts and see them, but if I duplicate a post title  to test my validation, it throws an exception: <code>SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry 'duplicate-slug' for key 'UNIQ_FAB8C3B3989D9B62'</code>. I have been scanning through documentation for quite some time now, but I was not able to find out why my <code>$form-&gt;isValid()</code> returns <code>true</code>.</p>