从文档中的子数组中查找最高值 [英] Finding highest value from sub-arrays in documents

问题描述

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 } ] }

我想运行一个返回具有最高V"的子文档的查询，所以在这种情况下我会得到:

I would like to run a query that returns the sub-document with the highest "V", so in that case I would get:

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

或者简单地说:

{ K: "E", V: 14 }

重要的部分是我希望 Mongo 服务器上的内存使用量为 O(1)(无论我处理多少个文档，内存使用量都是恒定的)，并且我只想用我需要的值(我不想下载不必要的子文档).

The important part is that I want the memory usage on the Mongo server to be O(1) (no matter how many documents I process, the memory usage is constant), and I only want to retrieve that one subdocument with the value I need (I don't want to download more subdocuments than necessary).

我的首选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，但不知道如何.我不希望将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).

My preferred approach would be to use a simple find query, but I'm not sure if that's possible. I suspect this can be also done with the aggregation framework (or map reduce?), but don't see how. I don't want the result stored in a temporary collection, but directly returned to my client (like a normal query).

推荐答案

下面的聚合集返回你所需要的.

The following aggratation set returns what you need.

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}}

享受吧！:)

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