请教 Lumen 的多对多关系中,涉及跨库问题的时候,中间表指定 connection 无效的问题

class Exam extends Model
{
    protected $connection = 'mysql';

    public function students()
    {
      return $this->belongsToMany(User::class)->using(ExamUser::class);
    }
}Copy

class User extends Model
{
  protected $connection = 'user_center';

  protected $table='productimportuser';
}
Copy

class ExamUser extends Pivot
{
    protected $connection = 'mysql';
}Copy

Base table or view not found: 1146 Table 'jcusercenter.exam_user' doesn't exist (SQL: select * from `exam_user` where `exam_id` = 12)Copy