对数组 mongodb 的索引进行排序 [英] Sorting on index of array mongodb

问题描述

我有一个集合，其中包含以下对象:

I have a collection where i have objects like:

{
    "_id" : ObjectId("5ab212249a639865c58b744e"),
   "levels" : [ 
        {
            "levelId" : 0,
            "siteId" : "5a0ff11dc7bd083ea6a706b1",
            "title" : "Hospital Services"
        }, 
        {
            "levelId" : 1,
            "siteId" : "5a0ff220c7bd083ea6a706d0",
            "title" : "Reference Testing"
        }, 
        {
            "levelId" : 2,
            "siteId" : "5a0ff24fc7bd083ea6a706da",
            "title" : "Des Moines(Reference Testing)"
        }
    ]
}

我想对级别数组的第二个对象的标题字段进行排序，例如levels.2.title
目前我的 mongo 查询看起来像:

I want to sort on the title field of 2nd object of levels array e.g. levels.2.title
Currently my mongo query looks like:

db.getCollection('5aaf63a69a639865c58b2ab9').aggregate([
{$sort : {'levels.2.title':1}}
])

但它没有给出预期的结果.
请帮忙.

But it is not giving desired results.
Please help.

推荐答案

您可以在 3.6 中尝试以下查询.

You can try below query in 3.6.

db.col.aggregate({$sort:{"levels.2.title":1}});

这种聚合和查找语义在 3.4 中是不同的.更多关于 jira 这里

This aggregation and find semantics are different in 3.4. More on jira here

所以

db.col.find().sort({"levels.2.title":1}) 

按预期工作，但聚合排序未按预期工作.

works as expected and aggregation sort is not working as expected.

在 3.4 中使用以下聚合.

Use below aggregation in 3.4.

使用$arrayElemAt在 $addFields 将计算值保留为文档中的额外字段，后跟 $sort 按字段排序.

Use $arrayElemAt to project the second element in $addFields to keep the computed value as the extra field in the document followed by $sort sort on field.

$project 排除排序字段以获得预期输出.

$project with exclusion to drop the sort field to get expected output.

db.col.aggregate([
 {"$addFields":{ "sort_element":{"$arrayElemAt":["$levels", 2]}}}, 
 {"$sort":{"sort_element.title":-1}},
 {"$project":{"sort_element":0}}
])

另外，您可以使用 $let 表达式直接输出$addFields 阶段.

Also, You can use $let expression to output the title field directly in $addFields stage.

db.col.aggregate([
 {"$addFields":{ "sort_field":{"$let:{"vars":{"ele":{$arrayElemAt":["$levels", 2]}}, in:"$$ele.title"}}}},      
 {"$sort":{"sort_field":-1}},
 {"$project":{"sort_field":0}}
])

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