Failing ZF2 validator requiring Service Locator

Unique.php

<?php

namespace My\Validator;

use Zend\Validator\AbstractValidator;
use Zend\ServiceManager\ServiceLocatorAwareInterface;
use Zend\ServiceManager\ServiceLocatorInterface;


class Unique extends AbstractValidator
    implements ServiceLocatorAwareInterface
{
    /**
     * Service Locator class
     * @var \Zend\ServiceManager\ServiceLocatorAwareInterface
     */
    protected $sm;


    public function isValid($value)
    {
        $sm = $this->getServiceLocator();
        var_dump($sm);
        $db = $sm->get('Zend\Db\Adapter\Adapter');
    }

    /**
     * Set service manager instance
     *
     * @param  ServiceLocatorInterface $serviceLocator
     * @return void
     */
    public function setServiceLocator(ServiceLocatorInterface $serviceLocator)
    {
        $this->sm = $serviceLocator;
    }

    /**
     * Retrieve service manager instance
     *
     * @return ServiceLocatorInterface
     */
    public function getServiceLocator()
    {
        return $this->sm;
    }
}

Hi - how did you solve this? :)