MongoDB匹配数组中最后一个子文档的某属性

说明

• 使用到的操作符$slice
• 文档说明

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"
        }
    ]
}