// コントローラでの使用例
public function index()
{
    $Product = $this->productRepository->find(1);
    dump($Product->maker_name);

    $Product->maker_name = 'あああ';
    $app['orm.em']->persist($Product);
    $app['orm.em']->flush();
    ...

    /**
     * @Method("GET")
     * @Route("/sample/{id}", name="sample_page")
     * @Template("Sample/index.twig")
     */
    public function sample($id)
    {
        /** @var Product $product */
        $product = $this->entityManager->getRepository('Eccube\Entity\Product')->find($id);
        dump($Product->maker_name);

        $Product->maker_name = 'あああ';
        $this->entityManager->persist($Product);    // ★ここでシステムエラー
        $this->entityManager->flush();

        return ['product' => $product];
    }