title: Symfony2 Validation Datetime 1 should be before Datetime 2
body: <p>I'm looking through the Symfony2 Validation Reference but I'm not finding what I need.</p>
  
  <p>I have a class Employment with a <strong>StartDate</strong> and <strong>EndDate</strong>.
  I would like to add an \@Assert() where it verifies that <em>StartDate is always BEFORE EndDate</em>.
  Is there a standard way of comparing class attributes as a Validation Constraint or should I create a custom validation constraint?</p>
  
  <pre><code>class Employment {
  
      /**
      * @ORM\Id
      * @ORM\Column(type="integer")
      * @ORM\GeneratedValue(strategy="AUTO")
      * @Expose() 
      */
      protected $id;
  
      /**
      * @ORM\Column(type="datetime") 
      * @Expose()
      * @Assert\DateTime()
      */
      protected $startDate;
  
      /**
      * @ORM\Column(type="datetime", nullable=TRUE)
      * @Expose()
      * @Assert\DateTime()
      */
      protected $endDate;
  
  ...
  }
  </code></pre>