mongo aggregate聚合之 $addFields

mongo聚合查询之 $addFields的作用：

大致说明：字面意思是添加字段，就是在查询的结果再添加一些字段信息，字段的内容自己决定，可能还是不能理解，下面上码

$addFields用法支持mongo3.4及以上的版本

1.这是字段统计式的用法

准备数据：

{
_id: 1,
student: "Maya",
homework: [ 10, 5, 10 ],
quiz: [ 10, 8 ],
extraCredit: 0
}
{
_id: 2,
student: "Ryan",
homework: [ 5, 6, 5 ],
quiz: [ 8, 8 ],
extraCredit: 8
}

执行命令：

db.scores.aggregate([
  {
    $addFields: {
      totalHomework: { $sum: "$homework" }, // 给每个查询结果添加totalHomework字段，值为原文档对象中homework字段数组的和

      totalQuiz: { $sum: "$quiz" }, // 同理，添加totalQuiz字段，值为quize数组字段的和
    },
  },
  {
    $addFields: {
      totalScore: { $add: ["$totalHomework", "$totalQuiz", "$extraCredit"] },
    }, // 再添加一个totalScore字段，值为上面的totalHomework，totalQuiz两个字段和原有字段extraCredit三个字段相加之和
  },
]);

结果：

{
    "_id" : 1,
    "student" : "Maya",
    "homework" : [ 10, 5, 10 ],
    "quiz" : [ 10, 8 ],
    "extraCredit" : 0,
    "totalHomework" : 25, // homework之和 10+5+10
    "totalQuiz" : 18, // quiz之和 10+8
    "totalScore" : 43 // totalHome+totalQuiz+extraCredit之和 25+18+0
}
{
    "_id" : 2,
    "student" : "Ryan",
    "homework" : [ 5, 6, 5 ],
    "quiz" : [ 8, 8 ],
    "extraCredit" : 8,
    "totalHomework" : 16,
    "totalQuiz" : 16,
    "totalScore" : 40
}

2.下面是给内嵌文档增加字段的用法:
准备数据：

{ _id: 1, type: "a", specs: { doors: 4} }
{ _id: 2, type: "b" }

执行命令：

db.vehicles.aggregate([
  {
    $addFields: {
      "specs.fuel_type": "unleaded", // 内嵌对象specs添加一个fuel_type字段,值为'unleaded'
    },
  },
]);

查询结果:

{ 
    _id: 1, 
    type: "a",
    specs: { doors: 4, fuel_type: "unleaded" } 
}
{ 
    _id: 2, 
    type: "b",
    specs: { fuel_type: "unleaded" } 
}

3.覆盖已有的字段:
准备数据：

{ _id: 1, dogs: 10, cats: 15 }

执行命令:

db.animals.aggregate([
  {
    $addFields: { cats: 20 },
  },
]);

结果：

{ _id: 1, dogs: 10, cats: 20 }

虽然是覆盖原有的字段，但并不会对数据做修改，大家放心

mongo官方原文链接：https://docs.mongodb.com/manual/reference/operator/aggregation/addFields/