用户id won't保存到数据库得到null

我在Symfony中有一对多的关系。因此，我建立自己的班次管理，为多个用户创建事件。

当我保存一个事件到数据库我得到user_id null，我不会保存它的登录用户，我想保存多个用户的事件。

所以我不想保存user_id的用户，我想保存事件，在var_dump我得到用户的用户id，但当它保存我得到null在我的数据库中。有人知道怎么解决这个问题吗?

类用户:

class User implements AdvancedUserInterface, 'Serializable
{
/**
 * @ORM'OneToMany(targetEntity="App'ShiftBundle'Entity'Shift", mappedBy="users", cascade={"all"})
 */
private $shifts;

    public function __construct()
    {
       $this->isActive = true;
       $this->isBlocked = false;
       $this->shifts = new 'Doctrine'Common'Collections'ArrayCollection();
    }

   /**
 * Add shifts
 *
 * @param 'App'ShiftBundle'Entity'Shift $shifts
 * @return User
 */
public function addShift('App'ShiftBundle'Entity'Shift $shifts)
{
    $this->shifts[] = $shifts;
    $shifts->setUsers($this);
    return $this;
}
/**
 * Remove shifts
 *
 * @param 'App'ShiftBundle'Entity'Shift $shifts
 */
public function removeShift('App'ShiftBundle'Entity'Shift $shifts)
{
    $this->shifts->removeElement($shifts);
}
/**
 * Get shifts
 *
 * @return 'Doctrine'Common'Collections'Collection 
 */
public function getShifts()
{
    return $this->shifts;
}
}

类转变

class Shift
{
/**
 * Set userId
 *
 * @param integer $userId
 * @return Shift
 */
public function setUserId($userId)
{
    $this->user_id = $userId;
    return $this;
}
/**
 * Get userId
 *
 * @return integer
 */
public function getUserId()
{
    return $this->user_id;
}
/**
* @ORM'ManyToOne(targetEntity="App'AuthBundle'Entity'User", inversedBy="shifts")
* @ORM'JoinColumn(name="user_id", referencedColumnName="id")
*/
private $users;
/**
 * Set users
 *
 * @param 'App'AuthBundle'Entity'User $users
 * @return Shift
 */
public function setUsers('App'AuthBundle'Entity'User $users)
{
    $this->users = $users;
    return $this;
}
/**
 * Get users
 *
 * @return 'App'AuthBundle'Entity'User 
 */
public function getUsers()
{
    return $this->users;
}
}

public function createShiftTimeAction(Request $request)
{
        $content = $request->getContent();
        $json = json_decode($content);
        $repository = $this->getDoctrine()->getRepository('AppShiftBundle:Shift');
        $shift = new Shift();
        var_dump($json->user_id);
        $shift->setDay($json->day);
        $shift->setMonth($json->month);
        $shift->setYear($json->year);
        $shift->setTimeFrom($json->time_from);
        $shift->setTimeTo($json->time_to);
        $shift->setUserId($json->user_id);
        $em = $this->getDoctrine()->getManager();
        $em->persist($shift);
        $em->flush();
        return new JsonResponse($json);
}