数组表达式运算符主要用于文档中数组的操作，接上一篇：

MongoDB 聚合管道中使用数组表达式运算符（$concatArrays合并数组）https://blog.csdn.net/m1729339749/article/details/130162048本篇我们主要介绍数组表达式运算符中用于断言数组的操作，断言数组的操作其目的是用于判断字段是否是数组，可以用于解决合并数组时我们可能会出现的问题，下面我们进行详细介绍：

一、语法

{ $isArray: <expression> }

        判断是否是数组

二、准备工作

初始化成员数据

db.persons.insertMany([
    { "_id" : "1001", "name" : "张三", "fruits" : [ "apple", "pineapple" ], "toys": [ "橡皮鸭" ] },
    { "_id" : "1002", "name" : "李四", "fruits" : [ "banana", "apple" ] },
    { "_id" : "1003", "name" : "王五", "fruits" : null, "toys": [ "小卡车" ] },
    { "_id" : "1004", "name" : "赵六", "fruits" : [ ], "toys": [ "毛绒鹅" ] },
    { "_id" : "1005", "name" : "田七", "toys": [ "积木" ] },
])

三、示例

例子1：判断fruits、toys是否是数组

db.persons.aggregate([
    {
        $project: {
            "name": 1,
            "fruitsIsArray": { $isArray: "$fruits" },
            "toysIsArray": { $isArray: "$toys" }
        }
    }
])

聚合查询的结果如下：

{ "_id" : "1001", "name" : "张三", "fruitsIsArray" : true, "toysIsArray" : true }
{ "_id" : "1002", "name" : "李四", "fruitsIsArray" : true, "toysIsArray" : false }
{ "_id" : "1003", "name" : "王五", "fruitsIsArray" : false, "toysIsArray" : true }
{ "_id" : "1004", "name" : "赵六", "fruitsIsArray" : true, "toysIsArray" : true }
{ "_id" : "1005", "name" : "田七", "fruitsIsArray" : false, "toysIsArray" : true }

从上面的结果可以看出，当字段不存在或者为null时，判断的结果为false

例子2：合并水果和玩具到喜欢的字段中

db.persons.aggregate([
    { 
        $addFields: {
            "favorite": {
                $cond: {
                    if: { $isArray: "$fruits" },
                    then: "$fruits",
                    else: []
                }
            }
        }
    },
    { 
        $addFields: {
            "favorite": {
                $cond: {
                    if: { $isArray: "$toys" },
                    then: { $concatArrays: [ "$favorite", "$toys" ] },
                    else: "$favorite"
                }
            }
        }
    }
])

聚合查询的结果如下：

{ "_id" : "1001", "name" : "张三", "fruits" : [ "apple", "pineapple" ], "toys" : [ "橡皮鸭" ], "favorite" : [ "apple", "pineapple", "橡皮鸭" ] }
{ "_id" : "1002", "name" : "李四", "fruits" : [ "banana", "apple" ], "favorite" : [ "banana", "apple" ] }
{ "_id" : "1003", "name" : "王五", "fruits" : null, "toys" : [ "小卡车" ], "favorite" : [ "小卡车" ] }
{ "_id" : "1004", "name" : "赵六", "fruits" : [ ], "toys" : [ "毛绒鹅" ], "favorite" : [ "毛绒鹅" ] }
{ "_id" : "1005", "name" : "田七", "toys" : [ "积木" ], "favorite" : [ "积木" ] }

通过上面的例子，我们成功解决了多个数组合并可能会出现的问题。