Constraints in Symfony

I have a class called Student with a StartDate and EndDate. I would like to add an \@Assert() where it verifies that StartDate is always BEFORE EndDate. This is what I have thus but the error message is not being executed. Can this be accomplished another way.

/**
 * @var \DateTime
 *
 * @ORM\Column(name="startDate", type="datetime", nullable=false)
 * @Assert\Type("DateTime")
 */
private $startdate;

/**
 * @var \DateTime
 *
 * @ORM\Column(name="endDate", type="datetime", nullable=false)
 * @Assert\Type("DateTime")
 * @Assert\Expression("this.getStartDate() < this.getEndDate()",
 * message="The end date must be after the start date")
 *
 * 
 */
private $enddate;

You could use a simple callback:

/**
 * @var \DateTime
 *
 * @ORM\Column(name="startDate", type="datetime", nullable=false)
 * @Assert\Type("DateTime")
 */
private $startdate;

/**
 * @var \DateTime
 *
 * @ORM\Column(name="endDate", type="datetime", nullable=false)
 * @Assert\Type("DateTime")
 * message="The end date must be after the start date")
 *
 * 
 */
private $enddate;

/**
 * @Assert\Callback
 */
public function validateDate(ExecutionContextInterface $context, $payload)
{
     if ($this->startdate > $this->enddate) {
        $context->buildViolation('Start date has to be before end date')
            ->atPath('startdate')
            ->addViolation();
    }
}

see https://symfony.com/doc/current/reference/constraints/Callback.html for details.