mongoose populate

mongoose populate用于对外键（objectid）字段的扩展，省去前段二次查询数据库的麻烦。

参考：http://mongoosejs.com/docs/populate.html

有2个特殊的用法：

1. 对objectid数组的populate，这个和单个objectid的populate同样；

2. Populating across multiple levels，即对objectid 下的objectid对象的populate

User. findOne({ name: 'Val' }).

populate({

    path: 'friends', // Get friends of friends - populate the 'friends' array for every friend populate:

     {    

         path: 'friends'

     }

}

);