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据上的转换，数组转换成对象，对象转换成数组，下面我们进行详细的介绍：

一、数组转对象（$arrayToObject）

如果想把一个数组转换成一个对象，数组需满足下面其中一个条件：

（1）这个数组是一个二维数组，数组中的元素必须是一个由两个元素组成的数组，第一个元素是对象的字段名称，第二个元素是对象的字段值；

        例如：[[ "name", "张三" ], [ "age", 10 ]]

                转换成{ "name": "张三",  "age": 10 }

（2）数组是一个对象数组，对象中包含了k和v两个字段，其中k用于存储字段名称，v用于存储字段值

        例如：[{ "k": "name", "v": "张三" }, { "k": "age", "v": 10 }]

                转换成 { "name": "张三", "age": 10 }

语法：{ $arrayToObject: <expression> }

        数组转换成对象

1、准备工作

初始化成员数据

db.persons.insertMany([{ "_id" : "1001", "name" : "张三", "age" : 20, "dept" : [ { "k" : "no", "v" : 1 }, { "k" : "dept", "v" : "财务部门" } ] },{ "_id" : "1002", "name" : "李四", "age" : 22, "dept" : [ { "k" : "no", "v" : 2 }, { "k" : "dept", "v" : "研发部门" } ] },{ "_id" : "1003", "name" : "王五", "age" : 30, "dept" : [ { "k" : "no", "v" : 2 }, { "k" : "dept", "v" : "研发部门" } ] }
])

2、示例

例子：部门转换成对象 

db.persons.aggregate([{$project: {"name": 1,"dept": { $arrayToObject: "$dept" }}}
])

聚合查询的结果如下：

{ "_id" : "1001", "name" : "张三", "dept" : { "no" : 1, "dept" : "财务部门" } }
{ "_id" : "1002", "name" : "李四", "dept" : { "no" : 2, "dept" : "研发部门" } }
{ "_id" : "1003", "name" : "王五", "dept" : { "no" : 2, "dept" : "研发部门" } }

二、对象转数组（$objectToArray）

语法：{ $objectToArray: <object> }

        对象转换成数组

返回值是由k和v组成的对象数组，其中k代表的是字段名称，v代表的是字段值。

1、准备工作

初始化成员数据

db.persons.insertMany([    { "_id" : "1001", "name" : "张三", "age" : 20, "dept": { "no": 1, "dept": "财务部门" } },{ "_id" : "1002", "name" : "李四", "age" : 22, "dept": { "no": 2, "dept": "研发部门" } },{ "_id" : "1003", "name" : "王五", "age" : 30, "dept": { "no": 2, "dept": "研发部门" } }
])

2、示例

例子：部门转换成数组

db.persons.aggregate([{$project: {"name": 1,"dept": { $objectToArray: "$dept" }}}
])

聚合查询的结果如下：

{ "_id" : "1001", "name" : "张三", "dept" : [ { "k" : "no", "v" : 1 }, { "k" : "dept", "v" : "财务部门" } ] }
{ "_id" : "1002", "name" : "李四", "dept" : [ { "k" : "no", "v" : 2 }, { "k" : "dept", "v" : "研发部门" } ] }
{ "_id" : "1003", "name" : "王五", "dept" : [ { "k" : "no", "v" : 2 }, { "k" : "dept", "v" : "研发部门" } ] }

三、综合案例

1、准备工作

初始化学生成绩

db.students.insertMany([{ "_id": 1, "name": "张三", "score": { "eng": 80, "math": 70 } },{ "_id": 2, "name": "李四", "score": { "eng": 60, "math": 90 } },{ "_id": 3, "name": "王五", "score": { "eng": 90, "math": 50 } }
])

2、示例

例子：计算学生的总成绩

第一步：将成绩转换成数组

db.students.aggregate([{$set: {"score": { $objectToArray: "$score" }}}
])

聚合查询的结果如下：

{ "_id" : 1, "name" : "张三", "score" : [ { "k" : "eng", "v" : 80 }, { "k" : "math", "v" : 70 } ] }
{ "_id" : 2, "name" : "李四", "score" : [ { "k" : "eng", "v" : 60 }, { "k" : "math", "v" : 90 } ] }
{ "_id" : 3, "name" : "王五", "score" : [ { "k" : "eng", "v" : 90 }, { "k" : "math", "v" : 50 } ] }

第二步： 使用$score.v求和，并合并到数组中

db.students.aggregate([{$set: {"score": { $objectToArray: "$score" }}}, {$set: {"score": { $concatArrays: [ "$score", [ { "k": "total", "v": { $sum: "$score.v" } } ] ]} }}
])

聚合查询的结果如下：

{ "_id" : 1, "name" : "张三", "score" : [ { "k" : "eng", "v" : 80 }, { "k" : "math", "v" : 70 }, { "k" : "total", "v" : 150 } ] }
{ "_id" : 2, "name" : "李四", "score" : [ { "k" : "eng", "v" : 60 }, { "k" : "math", "v" : 90 }, { "k" : "total", "v" : 150 } ] }
{ "_id" : 3, "name" : "王五", "score" : [ { "k" : "eng", "v" : 90 }, { "k" : "math", "v" : 50 }, { "k" : "total", "v" : 140 } ] }

 第三步： 将成绩数组转换成对象

db.students.aggregate([{$set: {"score": { $objectToArray: "$score" }}}, {$set: {"score": { $concatArrays: [ "$score", [ { "k": "total", "v": { $sum: "$score.v" } } ] ]} }},{$set: {"score": { $arrayToObject: "$score" }}}
])

聚合查询的结果如下：

{ "_id" : 1, "name" : "张三", "score" : { "eng" : 80, "math" : 70, "total" : 150 } }
{ "_id" : 2, "name" : "李四", "score" : { "eng" : 60, "math" : 90, "total" : 150 } }
{ "_id" : 3, "name" : "王五", "score" : { "eng" : 90, "math" : 50, "total" : 140 } }