Database Mongodb：如何；“展平”；一些查询结果

帮助在查询中“展平”（拉取与文档字段处于同一级别的嵌套字段）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"
        }
}
]

我的问题是：

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

您可以使用获取嵌套文档键和值，然后与一起使用动态连接值：

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" }
                        ]
                    }
                }
            }
        }
    }
])

---

$arrayToObject或$objectToArray？还有：为什么要用双美元符号

$$

？当访问

$$value

和

$$this.v

时，没有一个

$

？它也在文档中使用，但没有解释原因。单美元表示字段，双美元符号表示变量，

$reduce

引入两个变量

$$value

（状态）和

$$this

（当前处理的元素）

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" }
                        ]
                    }
                }
            }
        }
    }
])