mongoose populate

mongoose populate用於對外鍵（objectid）字段的擴展，省去前段二次查詢數據庫的麻煩。

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

有2個特殊的用法：

1. 對objectid數組的populate，這個和單個objectid的populate同樣；

2. Populating across multiple levels，即對objectid 下的objectid對象的populate

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

populate({

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

     {    

         path: 'friends'

     }

}

);