http://stackoverflow.com/questions/34253540/symfony-validato

    /**
     * @Callback()
     */
    public function validatePostalCode(ExecutionContextInterface $context)
    {
        $constraint = new ZipCode([
            'country' => $this->country,
        ]);
        $violations = $context->getValidator()->validate($this->postalCode, $constraint);

        foreach ($violations as $violation) {
            /* @var $violation ConstraintViolationInterface */
            $builder = $context->buildViolation($violation->getMessage(), $violation->getParameters());
            $builder->atPath('postalCode')
                ->setCode($violation->getCode())
                ->setInvalidValue($violation->getInvalidValue());

            // getPlural is only in ConstraintViolation prior to 3.0
            if (method_exists($violation, 'getPlural')) {
                $builder->setPlural($violation->getPlural());
            }

            // getCause is only in ConstraintViolation, not in ConstraintViolationInterface even in v3.0
            if (method_exists($violation, 'getCause')) {
                $builder->setCause($violation->getCause());
            }

            $builder->addViolation();
        }
    }