Unable to inject EntityManager into Twig extension - symfony

Question

I have a problem injecting the EntityManger into a Symfony2 twig-extension. I did the following:
I registered a new twig extension and created a class. All of it is working.
Now I wanted to get access to the database and tried to inject the doctrine EntityManager. My first step was to add the EM as parameter to the constructor:
use XYZ\BlubBundle\Utils\SessionHelper;
class SessionHelperExtension extends \Twig_Extension
{
private $em;
private $conn;
protected $sessionHelper;
public function __construct(\Doctrine\ORM\EntityManager $em, \XYZ\BlubBundle\Utils\SessionHelper $sessionHelper) {
$this->em = $em;
$this->conn = $em->getConnection();
$this->sessionHelper = $sessionHelper;
}
[...]
}
?>
Then I added the following lines to the service.yaml:
twig.extension.sessionHelper:
class: XYZ\BlubBundle\Extensions\SessionHelperExtension
arguments: [#session_helper, #doctrine.orm.entity_manager]
tags:
- { name: twig.extension }
My own Service "sessionHelper" gets injected without a problem, but the EntityManager is always "null". What am I doing wrong here?
EDIT:
I get the following exception:
Catchable Fatal Error: Argument 2 passed to
XYZ\BlubBundle\Extensions\SessionHelperExtension::__construct() must be an instance of
Doctrine\ORM\EntityManager, none given,
called in /.../Symfony/app/cache/dev/appDevDebugProjectContainer.php on line 2918 and
defined in /.../Symfony/src/XYZ/BlubBundle/Extensions/SessionHelperExtension.php line 12

Answer 1

public function __construct(
\Doctrine\ORM\EntityManager $em,
\XYZ\BlubBundle\Utils\SessionHelper $sessionHelper
) {
is wrong — you need to keep the same order like in the yml:
public function __construct(
\XYZ\BlubBundle\Utils\SessionHelper $sessionHelper,
\Doctrine\ORM\EntityManager $em
) {
Update
Try this: http://www.coderelic.com/2012/06/querying-the-database-from-a-twig-extension-in-symfony-2/