Symfony2: Overriding createAction() in SonataAdmin

I've been googling as crazy the last days trying to figure out (with no success) how override a SonataAdmin action to capture the session username and save it in the foreign key field.

AttachmentAdminController class:

<?php

namespace Application\Sonata\UserBundle\Controller;

use Sonata\AdminBundle\Controller\CRUDController as Controller;
#use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use FOS\UserBundle\Entity\User;
use Symfony\Component\Security\Core\SecurityContextInterface;
use Symfony\Bridge\Monolog\Logger;
use Mercury\CargoRecognitionBundle\Entity\Attachment;

class AttachmentAdminController extends Controller
{
    /**
     * (non-PHPdoc)
     * @see Sonata\AdminBundle\Controller.CRUDController::createAction()
     */
    public function createAction()
    {
        $result = parent::createAction();

        if ($this->get('request')->getMethod() == 'POST')
        {
            $flash = $this->get('session')->getFlash('sonata_flash_success');

            if (!empty($flash) && $flash == 'flash_create_success')
            {
                #$userManager = $this->container->get('fos_user.user_manager');
                #$user = $this->container->get('context.user');
                #$userManager = $session->get('username');
                $user = $this->container->get('security.context')->getToken()->getUser()->getUsername();

                $attachment = new Attachment();
                $attachment->setPath('/tmp/image.jpg');
                $attachment->setNotes('nothing interesting to say');
                $attachment->getSystemUser($user);

                $em = $this->getDoctrine()->getEntityManager();
                $em->persist($product);
                $em->flush();
            }
        }

        return $result;
    }
}

service attachment:

mercury.cargo_recognition.admin.attachment:
    class: Mercury\CargoRecognitionBundle\Admin\AttachmentAdmin
    tags:
        - { name: sonata.admin, manager_type: orm, group: General, label: Attachments }
    arguments: [ null, Mercury\CargoRecognitionBundle\Entity\Attachment, "SonataAdminBundle:CRUD" ]

Seems to me as the actionController() is been ignored by SonataAdminBundle (and maybe the whole class file), because there's not error messages at all, but I don't know why. Actually, I'm not sure if I'm fetching the username from the session.

I really need a good tutorial about this, but seems like any information I get about this is obsolete in some aspect. By the way, I'm using Symfony 2.0.16