InvitationToCodeTransformer.php

<?php

namespace NSM\UserBundle\Form\DataTransformer;

use NSM\UserBundle\Entity\Invitation;
use Doctrine\ORM\EntityManager;
use Symfony\Component\Form\DataTransformerInterface;
use Symfony\Component\Form\Exception\UnexpectedTypeException;

/**
 * Transforms an Invitation to an invitation code.
 */
class InvitationToCodeTransformer implements DataTransformerInterface
{
    protected $entityManager;

    public function __construct(EntityManager $entityManager)
    {
        $this->entityManager = $entityManager;
    }

    public function transform($value)
    {
        // Generally I wouldn't have to check for false
        // But false is set in the reverse transform
        if (null === $value || false === $value) {
            return null;
        }

        if (!$value instanceof Invitation) {
            throw new UnexpectedTypeException($value, 'NSM\UserBundle\Entity\Invitation');
        }

        return $value->getCode();
    }

    public function reverseTransform($value)
    {
        if (null === $value || '' === $value) {
            return null;
        }

        if (!is_string($value)) {
            throw new UnexpectedTypeException($value, 'string');
        }

        // Try and grab the invite
        $invite = $this->entityManager
            ->getRepository('NSM\UserBundle\Entity\Invitation')
            ->findOneBy(array(
                'code' => $value,
                'user' => null,
            )
        );

         // return false if there is no entity otherwise return the entity
         return (null === $invite) ? false : $invite;
    }
}

InviteValidator.php

<?php

namespace NSM\UserBundle\Validator\Constraints;

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

class InviteValidator extends ConstraintValidator
{
    public function validate($value, Constraint $constraint)
    {

        if(false === $value) {
            $this->context->addViolation($constraint->notFoundMessage, array('%string%' => $value));
            return false;
        }

        // Check expiration
        // Check assignment to email

        return true;
    }
}