Doctrine ManyToMany总是返回一个空集合

Question

我有这两个类与ManyToMany关联：

一堂课：

/**
 * @ORM\ManyToMany(targetEntity="Application\Sonata\UserBundle\Entity\User", inversedBy="tripEvents")
 * @ORM\JoinTable(name="event_trip_registrators")
 */
private $tripRegistrators;

public function __construct()
{
    $this->tripRegistrators = new ArrayCollection();
}

public function getTripRegistrators()
{
    return $this->tripRegistrators;
}

public function setTripRegistrators($tripRegistrators)
{
    $this->tripRegistrators = $tripRegistrators;
}

public function addTripRegistrator(User $tripRegistrator)
{
    $this->tripRegistrators->add($tripRegistrator);
}

public function removeTripRegistrator($tripRegistrator)
{
    $this->tripRegistrators->removeElement($tripRegistrator);
}

第二课：

/**
 * @ORM\ManyToMany(targetEntity="Bundle\Entity\Event", mappedBy="tripRegistrators")
 */
protected $tripEvents;

public function __construct()
{
    parent::__construct();
    $this->tripEvents = new ArrayCollection();
}

public function getTripEvents()
{
    return $this->tripEvents;
}

public function setTripEvents($tripEvents)
{
    $this->tripEvents = $tripEvents;
}

如果我调用$event->getTripRegistrators()（第一类），我只会得到一个空的持久集合。 你有任何暗示为什么会这样吗？ 如果我通过SonataAdmin保存项目，一切正常，数据库表有正确的数据。