Auth Storage adapter for Zend Framework and Doctrine 2

Session.php

<?php
namespace Ocramius\Auth\Storage;

use Entity\Ocramius\Admin as AdminEntity,
    Doctrine\ORM\EntityManager;

/**
 * An @see \Zend_Auth_Storage_Session that handles @see AdminEntity items
 * @author Marco Pivetta <ocramius@gmail.com>
 */
class Session extends \Zend_Auth_Storage_Session {
    
    const ADMIN_NAMESPACE_DEFAULT = 'admin_session_storage';
    
    /**
     * @var EntityManager
     */
    protected $_entityManager;
    
    /**
     * @param EntityManager $entityManager
     * @param string $namespace
     * @param string $member 
     */
    public function __construct(
        EntityManager $entityManager,
        $namespace = self::ADMIN_NAMESPACE_DEFAULT,
        $member = self::MEMBER_DEFAULT
    ) {
        $this->_entityManager = $entityManager;
        parent::__construct($namespace, $member);
    }
    
    public function isEmpty() {
        return parent::isEmpty() || $this->read() === null;
    }
    
    /**
     * @param AdminEntity|null $contents 
     */
    public function write($contents) {
        if($contents instanceof AdminEntity) {
            $identifier = $this
                ->_entityManager
                ->getClassMetadata(AdminEntity::getClassName())
                ->getIdentifierValues($contents);
            $contents = empty($identifier) ? null : $identifier;
        }else if($contents !== null) {
            throw new InvalidArgumentException(
                'Persisted identity must be either an '
                . AdminEntity::getClassName() . ' or null'
            );
        }
        parent::write($contents);
    }
    
    /**
     * @return AdminEntity|null
     */
    public function read() {
        $data = parent::read();
        if(empty($data)) {
            return null;
        }
        $data = $this
            ->_entityManager
            ->find(AdminEntity::getClassName(), $data);
        if($data === null) {
            parent::write(null);
        }
        return $data;
    }
    
}