MongoDB 聚合管道中使用数组表达式运算符（$slice截取数组）

数组表达式运算符主要用于文档中数组的操作，本篇我们主要介绍数组表达式运算符中用于截取数组的操作，下面我们进行详细介绍：

一、语法

{ $slice: [ , ] }

或者 { $slice: [ , , ] }

        获取数组中指定范围内的元素组成一个新的数组

：代表的是数组

：代表的是起始位置索引（包含此位置的元素）

：代表的是获取N个元素

二、准备工作

初始化成员数据

db.persons.insertMany([
    { "_id" : "1001", "name" : "张三", "fruits" : [ "apple", "pineapple", "orange" ] },
    { "_id" : "1002", "name" : "李四", "fruits" : [ "banana", "apple", "pineapple" ] },
    { "_id" : "1003", "name" : "王五", "fruits" : [ "banana", "apple", "orange", "watermelon" ] },
    { "_id" : "1004", "name" : "赵六", "fruits" : [ ] },
    { "_id" : "1005", "name" : "田七" },
])

三、示例

例子1：找到每个人最喜欢吃的前两个水果

db.persons.aggregate([
    {
        $project: {
            "name": 1,
            "favoriteFruits": { $slice: [ "$fruits", 2 ] }
        }
    }
])

聚合查询的结果如下：

{ "_id" : "1001", "name" : "张三", "favoriteFruits" : [ "apple", "pineapple" ] }
{ "_id" : "1002", "name" : "李四", "favoriteFruits" : [ "banana", "apple" ] }
{ "_id" : "1003", "name" : "王五", "favoriteFruits" : [ "banana", "apple" ] }
{ "_id" : "1004", "name" : "赵六", "favoriteFruits" : [ ] }
{ "_id" : "1005", "name" : "田七", "favoriteFruits" : null }

例子2：找到每个人最喜欢吃的最后两个水果

db.persons.aggregate([
    {
        $project: {
            "name": 1,
            "favoriteFruits": { $slice: [ "$fruits", -2 ] }
        }
    }
])

聚合查询的结果如下：

{ "_id" : "1001", "name" : "张三", "favoriteFruits" : [ "pineapple", "orange" ] }
{ "_id" : "1002", "name" : "李四", "favoriteFruits" : [ "apple", "pineapple" ] }
{ "_id" : "1003", "name" : "王五", "favoriteFruits" : [ "orange", "watermelon" ] }
{ "_id" : "1004", "name" : "赵六", "favoriteFruits" : [ ] }
{ "_id" : "1005", "name" : "田七", "favoriteFruits" : null }

（1）字段未定义时，结果为null；

（2）未找到元素，结果为[ ]

（3）{ $slice: [ , ] } 语法中

                n为正数，代表的是从前往后取N个元素

                n为负数，代表的是从后往前取N个元素

例子3：找到每个人最喜欢吃的第二个水果

db.persons.aggregate([
    {
        $project: {
            "name": 1,
            "favoriteFruits": { $slice: [ "$fruits", 1, 1 ] }
        }
    }
])

聚合查询的结果如下：

{ "_id" : "1001", "name" : "张三", "favoriteFruits" : [ "pineapple" ] }
{ "_id" : "1002", "name" : "李四", "favoriteFruits" : [ "apple" ] }
{ "_id" : "1003", "name" : "王五", "favoriteFruits" : [ "apple" ] }
{ "_id" : "1004", "name" : "赵六", "favoriteFruits" : [ ] }
{ "_id" : "1005", "name" : "田七", "favoriteFruits" : null }

例子4：假如position为负数，会如何处理

db.persons.aggregate([
    {
        $project: {
            "name": 1,
            "favoriteFruits": { $slice: [ "$fruits", -2, 1 ] }
        }
    }
])

聚合查询的结果如下：

{ "_id" : "1001", "name" : "张三", "favoriteFruits" : [ "pineapple" ] }
{ "_id" : "1002", "name" : "李四", "favoriteFruits" : [ "apple" ] }
{ "_id" : "1003", "name" : "王五", "favoriteFruits" : [ "orange" ] }
{ "_id" : "1004", "name" : "赵六", "favoriteFruits" : [ ] }
{ "_id" : "1005", "name" : "田七", "favoriteFruits" : null }

例子5：假如position = -10，会如何处理

db.persons.aggregate([
    {
        $project: {
            "name": 1,
            "favoriteFruits": { $slice: [ "$fruits", -10, 1 ] }
        }
    }
])

聚合查询的结果如下：

{ "_id" : "1001", "name" : "张三", "favoriteFruits" : [ "apple" ] }
{ "_id" : "1002", "name" : "李四", "favoriteFruits" : [ "banana" ] }
{ "_id" : "1003", "name" : "王五", "favoriteFruits" : [ "banana" ] }
{ "_id" : "1004", "name" : "赵六", "favoriteFruits" : [ ] }
{ "_id" : "1005", "name" : "田七", "favoriteFruits" : null }

如果position为负数，则从后往前确定起始位置，当向前移动的位置超过了数组长度，则起始位置为0；然后再从前往后获取N个元素组成新的数组。