Validator constraint with dependencies

LaterDateField.php

<?php

namespace My\Bundle\Validator\Constraint;

use Symfony\Component\Validator\Constraint;

/**
 * @Annotation
 */
class LaterDateField extends Constraint
{
    public $message = 'This date has to be later than {{ field }}.';
    public $field;

    public function getDefaultOption()
    {
        return 'field';
    }

    public function getRequiredOptions()
    {
        return array('field');
    }
}

LaterDateFieldValidator.php

<?php

namespace My\Bundle\Validator\Constraint;

use Symfony\Component\Validator\ConstraintValidator;
use Symfony\Component\Validator\Constraint;
use Symfony\Component\Validator\Exception\UnexpectedTypeException;

use DateTime;

class LaterDateFieldValidator extends ConstraintValidator
{
    public function isValid($value, Constraint $constraint)
    {
        $earlier = $this->context->getRoot()->get($constraint->field)->getData();
        $later = $value;

        if (null !== $earlier && !$earlier instanceof DateTime) {
            throw new UnexpectedTypeException($earlier, 'DateTime');
        }

        if (null !== $later && !$later instanceof DateTime) {
            throw new UnexpectedTypeException($later, 'DateTime');
        }

        if ($earlier > $later) {
            $this->setMessage($constraint->message, array(
                '{{ field }}' => $constraint->field,
            ));
            return false;
        }

        return true;
    }
}