Question

假设我有以下收藏：

{ _id: 1, Array: [
  { K: "A", V: 8 },
  { K: "B", V: 5 },
  { K: "C", V: 13 } ] }

{ _id: 2, Array: [
  { K: "D", V: 12 },
  { K: "E", V: 14 },
  { K: "F", V: 2 } ] }

我想运行一个返回最高“V”的子文档的查询，所以在这种情况下我会得到：

{ _id: 1, Array: [ { K: "E", V: 14 } ] }

或简单地说：

{ K: "E", V: 14 }

重要的是我希望Mongo服务器上的内存使用量为O（1）（无论我处理多少文件，内存使用量是不变的），我只想检索那个子文档我需要的值（我不想下载超过必要的子文档）。

我首选的方法是使用简单的查找查询，但我不确定这是否可行。我怀疑这也可以通过聚合框架（或map reduce？）来完成，但是看不出来。我不希望结果存储在临时集合中，而是直接返回给我的客户端（就像普通查询一样）。

Answer 1

以下聚合集返回您需要的内容。

db.letters.aggregate([
    {$project:{"Array.K":1, "Array.V":1}},
    {$unwind:"$Array"},
    {$sort:{"Array.V":-1}},
    {$limit:1}
]);

返回：

{"_id":2, "Array":{"K":"E","V":14}}

享受！：）

Answer 2

正如@JohnnyHK所说：

db.col.aggregate([
    {$unwind: '$Array'},
    {$group: {_id: '$_id', Array: {K: {$max: '$K'}, V: {$max: '$V'}}}}
])

类似的东西。

Answer 3

IN Simple Words ，如果您有 mongo查询响应，如下所示 - 并且您只需要来自 Array-＆gt;的最高值＆＃34; Wish_CreatedDate＆＃34;

{ "_id": "57ee5a708e117c754915a2a2", "TotalWishs": 3, "Events": [ "57f805c866bf62f12edb8024" ], "wish": [ "Cosmic Eldorado Mountain Bikes, 26-inch (Grey/White)", "Asics Men's Gel-Nimbus 18 Black, Snow and Fiery Red Running Shoes - 10 UK/India (45 EU) (11 US)", "Suunto Digital Black Dial Unisex Watch - SS018734000" ], "Wish_CreatedDate": [ "2017-03-05T00:00:00.000Z", "2017-02-13T00:00:00.000Z" ], "UserDetails": [ { "createdAt": "2016-09-30T12:28:32.773Z", "jeenesFriends": [ "57edf8a96ad8f6ff453a384a", "57ee516c8e117c754915a26b", "58a1644b6c91d2af783770b0", "57ef4631b97d81824cf54795" ], "userImage": "user_profile/Male.png", "email": "roopak@small-screen.com", "fullName": "Roopak Kapoor" } ], },

***然后你添加了

Latest_Wish_CreatedDate：{$ max：＆＃34; $ Wish_CreatedDate＆＃34;}，

如下所示 -

{ $project : { _id: 1, TotalWishs : 1 , wish:1 , Events:1, Wish_CreatedDate:1, Latest_Wish_CreatedDate: { $max: "$Wish_CreatedDate"}, } }

最终查询响应将低于

{ "_id": "57ee5a708e117c754915a2a2", "TotalWishs": 3, "Events": [ "57f805c866bf62f12edb8024" ], "wish": [ "Cosmic Eldorado Mountain Bikes, 26-inch (Grey/White)", "Asics Men's Gel-Nimbus 18 Black, Snow and Fiery Red Running Shoes - 10 UK/India (45 EU) (11 US)", "Suunto Digital Black Dial Unisex Watch - SS018734000" ], "Wish_CreatedDate": [ "2017-03-05T00:00:00.000Z", "2017-02-13T00:00:00.000Z" ], "UserDetails": [ { "createdAt": "2016-09-30T12:28:32.773Z", "jeenesFriends": [ "57edf8a96ad8f6ff453a384a", "57ee516c8e117c754915a26b", "58a1644b6c91d2af783770b0", "57ef4631b97d81824cf54795" ], "userImage": "user_profile/Male.png", "email": "roopak@small-screen.com", "fullName": "Roopak Kapoor" } ], "Latest_Wish_CreatedDate": "2017-03-05T00:00:00.000Z" },

从文档中的子数组中查找最高值

3 个答案: