Question

我有2个收藏如下

作者：

   {id:'1',name:'JK', bookCount:3}
   {id:'2',name:'SS', bookCount:2}
   {id:'3',name:'CC', bookCount:4}

预订 :( authorId：作者集合的外键）

   {name:'Book 1',authorId:1}
   {name:'Book 2',authorId:1}
   {name:'Book 3',authorId:1}
   {name:'Book 4',authorId:2}
   {name:'Book 5',authorId:2}
   {name:'Book 6',authorId:3}
   {name:'Book 7',authorId:3}
   {name:'Book 8',authorId:3}
   {name:'Book 9',authorId:3}

我想写一个mongo查询，它提供书籍，按 - ＆gt;排序相应作者按升序编写的书籍数量，即

   {name:'Book 4',authorId:2}
   {name:'Book 5',authorId:2}
   {name:'Book 1',authorId:1}
   {name:'Book 2',authorId:1}  
   {name:'Book 3',authorId:1}
   {name:'Book 6',authorId:3}
    .......

我应该怎么做呢。 Thanx提前！：）

Answer 1

根据您的输出，您应该只需要聚合 Book 集合和聚合查询，如下所示：

db.book.aggregate({
  "$group": {
    "_id": "$authorId",
    "names": {
      "$push": "$name"
    },
    "count": {
      "$sum": 1
    }
  }
}, {
  "$unwind": "$names"
}, {
  "$sort": {
    "count": 1
  }
}, {
  "$project": {
    "name": "$names",
    "authorId": "$_id",
    "_id": 0
  }
})

Answer 2

步骤：

1）根据authorId对您的数据进行分组并计算和还制作一个包含该组中所有书籍的数组

2）展开书籍数组

3）根据您在分组期间创建的计数字段进行排序

4）最终以您需要的格式投影。

db.book.aggregate([
{"$group": { "_id" : "$authorId", "count": {"$sum" : 1}, 
  "arr" :   {"$push" : "$name" }}
},
{"$unwind" :"$arr"},
{"$sort":{"$count": 1 }},
{"$project": {"name" : "$arr", "authorId" : "$_id"}
}
 ]);

通过映射集合来订购记录

2 个答案: