title: Symfony entity validation won't report violation
body: <p>I am trying to add some validation to my entity, using the Symfony validation component, i have added some constraints to my User Entity.</p>
  
  <pre><code>/**
   * @param ClassMetadata $metadata
   */
  public static function loadValidatorMetadata(ClassMetadata $metadata)
  {
      $metadata-&gt;addPropertyConstraint('username', new Assert\NotBlank);
      $metadata-&gt;addPropertyConstraint('password', new Assert\NotBlank);
      $metadata-&gt;addPropertyConstraint('first_name', new Assert\NotBlank);
      $metadata-&gt;addPropertyConstraint('last_name', new Assert\NotBlank);
  }
  </code></pre>
  
  <p>Now i want to test if i get some errors, when violating the constraints, this is done like this.</p>
  
  <pre><code>    $user = new User();
      $user-&gt;username = '';
      $user-&gt;password = '';
  
      $validator = Validation::createValidator();
  
      if (0 &lt; count($validator-&gt;validate($user))) {
          throw new \RuntimeException('The given user is invalid');
      }
  </code></pre>
  
  <p>But the count is zero, which is odd, as all the constraints is clearly violated? Am i missing something here? Well i must be :D.</p>
  
  <p>Might be worth to notice, that my application is not a Symfony application; it's a ordinary php application, i am just using the component.</p>