PHP 依赖注入和继承

嗨，关于依赖注入的快速问题

我正在使用symfony 3并且正在接受DI

假设我有一堂课

use Doctrine'ORM'EntityManagerInterface;
class CommonRepository
{
    /**
    * @var EntityManagerInterface
    */
    protected $em;
    /**
    * DoctrineUserRepository constructor.
    * @param EntityManagerInterface $em
    */
    public function __construct(EntityManagerInterface $em)
    {
        $this->em = $em;
    }
    public function getEntityManager()
    {
        return $this->em;
    }
}

现在一个名为UserRepository的新类，我注入了上面的类，这是否意味着我可以访问注入的项目注入的项目（显然他在开始结束时做梦）？

class UserRepository
{
    /**
     * @var CommonDoctrineRepository
     */
    private $commonRepository;
    /**
     * @var 'Doctrine'ORM'EntityManagerInterface
     */
    private $em;
    /**
    * DoctrineUserRepository constructor.
    * @param CommonRepository $CommonRepository
    */
    public function __construct(CommonRepository $CommonRepository)
    {
        $this->commonRepository = $commonRepository;
        $this->em = $this->commonRepository->getEntityManager();
    }
    public function find($id)
    {
        //does not seem to work
        //return $em->find($id);
        //nor does 
        //return $this->em->find($id);
    }
}

即使我扩展班级，然后试图构建父母没有快乐，显然我可以注入教义经理进入用户存储库，我只是想对 DI 和继承有所了解

class UserRepository extends CommonRepository 
{
    /**
     * @var CommonDoctrineRepository
     */
    private $commonRepository;
    /**
     * @var 'Doctrine'ORM'EntityManagerInterface
     */
     private $em;
    public function __construct(CommonDoctrineRepository $commonRepository, $em)
    {
        parent::__construct($em);
        $this->commonRepository = $commonRepository;
    }
}

对于symfony组件，我已经定义了这样的服务

app.repository.common_repository:
    class: AppBundle'Repository'Doctrine'CommonRepository
    arguments:
        - "@doctrine.orm.entity_manager"
app.repository.user_repository:
    class: AppBundle'Repository'Doctrine'UserRepository
    arguments:
        - "@app.repository.common_repository"