MongoDb 查询以获取数组内字段的最大值 [英] MongoDb query to get max of field inside array

问题描述

如何在下面的文档中获取最大的sections.Id，其中collection._id = some parameter

How to get the maximum of sections.Id in below document where collection._id = some parameter

{
    "_id" : ObjectId("571c5c87faf473f40fd0317c"),
    "name" : "test 1",
    "sections" : [ 
        {
             "Id" : 1,
             "name" : "first section"
        }, 
        {
            "Id" : 2,
            "name" : "section 2"
        }, 
        {
            "Id" : 3,
            "name" : "section 3"
        }
}

我在下面试过

db.collection.aggregate(
[
    {
        "$match": {
            "_id": ObjectId("571c5c87faf473f40fd0317c")
        }
    },
    {
        "$group" : {
            "_id" : "$_id",
            "maxSectionId" : {"$max" : "$sections.Id"}
        }
    }
]);

但不是返回 max int 单个值，而是返回部分数组中所有 Id 的数组.

But instead of returning max int single value it is returning an array of all Ids in sections array.

在 node.js 中执行进一步相同的查询时，它返回一个空数组.

Further same query when executed in node.js it returns an empty array.

推荐答案

您的聚合查询需要 $unwind for opennig to "sections" 数组

your aggregation query need $unwind for opennig to "sections" array

添加您的聚合查询

{$unwind : "$sections"}

和你这样的重构聚合查询

and your refactoring aggregation query like this

db.collection.aggregate(
[  
    {$unwind : "$sections"},
    {
        "$match": {
            "_id": ObjectId("571c5c87faf473f40fd0317c")
        }
    },
    {
        "$group" : {
            "_id" : "$_id",
            "maxSectionId" : {"$max" : "$sections.Id"}
        }
    }
]);

以及关于 $unwind 的更多知识:https://docs.mongodb.org/manual/reference/operator/aggregation/unwind/

and more knowledge for $unwind : https://docs.mongodb.org/manual/reference/operator/aggregation/unwind/

这篇关于MongoDb 查询以获取数组内字段的最大值的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！