Laravel父/子关系在它自己的模型上

我要获取至少有一个子节点的所有代金券，一张代金券可以有多个子节点，但任何代金券只能有一个父节点。

我用以下模型和调用设置了它，它生成的查询是所需的，直到这一部分:'vouchers'.'parent_id' = 'vouchers'.'id'

想要的功能:

$vouchers = Voucher::has('children')->get();

或

$vouchers = Voucher::has('parent')->get();

导致查询

select * from `vouchers` where `vouchers`.`deleted_at` is null 
and (select count(*) from `vouchers` where `vouchers`.`deleted_at` is null 
and `vouchers`.`parent_id` = `vouchers`.`id` 
and `vouchers`.`deleted_at` is null ) >= 1

模型:

class Voucher extends baseModel {
    public function parent()
    {
        return $this->belongsTo('Voucher', 'parent_id');
        // return $this->belongsTo('Voucher', 'parent_id', 'id'); <- attempted but din't work
    }
    public function children()
    {
        return $this->hasMany('Voucher', 'parent_id');
    }
}