MongoDB匹配数组中最后一个子文档的某属性

男娘i 2022-05-12 16:28 144阅读 0赞

### 说明 ###

*  使用到的操作符`$slice`
 *  [文档说明][Link 1]

### 示例 ###

#### 插入数据 ####

db.users.insert({ "books":[{ "name":"spring","author":"manA"},{ "name":"summer","author":"manB"},{ "name":"autumn","author":"manC"},{ "name":"winter","author":"manD"}]})
    
    db.users.insert({ "books":[{ "name":"spring","author":"manE"},{ "name":"summer","author":"manF"},{ "name":"autumn","author":"manG"},{ "name":"winter","author":"manH"}]})
    
    db.users.insert({ "books":[{ "name":"spring","author":"manI"},{ "name":"summer","author":"manJ"},{ "name":"autumn","author":"manK"},{ "name":"winter","author":"manL"}]})

#### 检索过滤 ####

*  语法：`{$slice: [startIndex, length]}`

db.users.find({ },{ "books":{ $slice:[2,2]}})

*  结果

{  
        "_id" : ObjectId("5bcfd2be7d1ec2d48d33f98f"), 
        "books" : [
            { 
                "name" : "autumn", 
                "author" : "manC"
            }, 
            { 
                "name" : "winter", 
                "author" : "manD"
            }
        ]
    }
    {  
        "_id" : ObjectId("5bcfd3f77d1ec2d48d33f990"), 
        "books" : [
            { 
                "name" : "autumn", 
                "author" : "manG"
            }, 
            { 
                "name" : "winter", 
                "author" : "manH"
            }
        ]
    }
    {  
        "_id" : ObjectId("5bcfd4317d1ec2d48d33f991"), 
        "books" : [
            { 
                "name" : "autumn", 
                "author" : "manK"
            }, 
            { 
                "name" : "winter", 
                "author" : "manL"
            }
        ]
    }

#### 聚合过滤（匹配数组中最后一个子文档的某属性） ####

*  说明：先过滤，后查询
 *  语法：`$slice:["$array", [startIndex, ] length ]` （startIndex可以省略，默认从0开始）
 *  属性名 `books`可以更改，进行输出重命名

db.users.aggregate({ $project:{ "books":{ $slice:["$books", -1]}}},{ $match:{ "books.author":"manD"}})

*  结果

{  
        "_id" : ObjectId("5bcfd2be7d1ec2d48d33f98f"), 
        "books" : [
            { 
                "name" : "winter", 
                "author" : "manD"
            }
        ]
    }

[Link 1]: https://docs.mongodb.com/manual/reference/operator/aggregation/slice/