多对多关系的新增问题.

有几个多对多关系的表 need, part, need_part
新增后，need_part表中的need_id和part_id都是正常的， 就是没有name。
入库后结果:

以下是代码， 不知道哪里有问题，请大大们帮看~~

有三张表:

need: ( id )
part: (id, name)
need_part: ( id, need_id, part_id, name )

我的三个模型:

Need.php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Need extends Model
{
    public function parts()
    {
        return $this->belongsToMany('App\Models\Part');
    }
}

Part.php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Part extends Model
{
    protected $table = 'part';

    public function needs()
    {
        return $this->belongsToMany('App\Models\Need');
    }

}

NeedPart.php

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class NeedPart extends Model
{

    protected $table = 'need_part';
    public $timestamps = false;

}

控制器中的新增代码:

$parts = Part::whereIn('id', [1,2,3])->get();
$need = Need::find(2);
$need->parts()->saveMany($parts);