Mongodb：如何“扁平化”一些查询结果答案

【问题标题】：Mongodb: how to "flatten" some query resultsMongodb：如何“扁平化”一些查询结果
【发布时间】：2020-06-12 23:56:10
【问题描述】：

帮助在查询中“扁平化”（将嵌套字段与文档字段拉到同一级别）一个 mongodb 文档

//this is "anagrafiche" collection
[{
 "name": "tizio"
,"surname": "semproni"
,"birthday": "01/02/1923"
,"home": {
     "road": "via"
    ,"roadname": "bianca"
    ,"roadN": 12
        ,"city": "rome"
        ,"country": "italy"
        }
},
{
 "name": "caio"
,"surname": "giulio"
,"birthday": "02/03/1932"
,"home": {
     "road": "via"
    ,"roadname": "rossa"
    ,"roadN": 21
        ,"city": "milan"
        ,"country": "italy"
        }
},
{
 "name": "mario"
 ,"surname": "rossi"
// birthday is not present for this document
,"home": {
     "road": "via"
    ,"roadname": "della pace"
    ,"roadN": 120
        ,"city": "rome"
        ,"country": "italy"
        }
}
]

我的查询：

db.anagrafiche.aggregate([  {$match {"home.city": "rome"}}
                {$project:{"name": 1, "surname":1, <an expression to flatten the address>, "birthday": 1, "_id":0}}
             ]
);

预期结果：

{
    ,"name": "tizio"
    ,"surname": "semproni"
    ,"address": "via bianca 12 rome"
    ,"birthday": 01/02/1923
},{
    ,"name": "mario"
    ,"surname": "rossi"
    ,"address": "via della pace 120 rome"
    ,"birthday": NULL
}

【问题讨论】：

标签： database mongodb aggregation-framework

【解决方案1】：

您可以使用$objectToArray 获取嵌套的文档键和值，然后使用$reduce 和$concat 动态连接值：

db.collection.aggregate([
    {
        $project: {
            _id: 0,
            name: 1,
            surname: 1,
            birthday: 1,
            address: {
                $reduce: {
                    input: { $objectToArray: "$home" },
                    initialValue: "",
                    in: {
                        $concat: [
                            "$$value",
                            { $cond: [ { $eq: [ "$$value", "" ] }, "", " " ] },
                            { $toString: "$$this.v" }
                        ]
                    }
                }
            }
        }
    }
])

Mongo Playground

【讨论】：

$arrayToObject 还是 $objectToArray？
另外：为什么是双美元符号$$？访问$$value 和$$this.v 时没有一个$？它也在文档中使用，但没有解释原因。
单美元指一个字段，双美元指一个变量，$reduce引入两个变量$$value（状态）和$$this（当前处理的元素）