MongoDB 聚合有两个模型

Posted 2023-03-11

【中文标题】MongoDB 聚合有两个模型

【英文标题】：MongoDB aggregate with two model

【发布时间】：2021-09-05 01:46:18

【问题描述】：

我有两个系列， 班级收藏和学生收藏

class collection

    "_id" : "1999558581",
    "name" : "Class One",
,

    "_id" : "1999558582",
    "name" : "Class Two",
,

    "_id" : "1999558583",
    "name" : "Class Three",

和

student collection

    "_id" : "111111",
    "First name" : "",
    "Last name" : "",
    "gender" : "Male",
    "class" : "1999558581", //classObjectId
,

    "_id" : "111112",
    "First name" : "",
    "Last name" : "",
    "gender" : "Female",
    "class" : "1999558581", //classObjectId
,

    "_id" : "111113",
    "First name" : "",
    "Last name" : "",
    "gender" : "Male",
    "class" : "1999558582", //classObjectId
,

    "_id" : "111114",
    "First name" : "",
    "Last name" : "",
    "gender" : "Male",
    "class" : "1999558583", //classObjectId
,

    "_id" : "111115",
    "First name" : "",
    "Last name" : "",
    "gender" : "Female",
    "class" : "1999558581", //classObjectId

我想在一个类中按性别获取聚合，并在类集合中获取名称。

#这是我试过的

 const male = await models.Student.aggregate([
         $match:  gender: "Male"  ,
         $group:  _id: "$class", count:  $sum: 1   ,
    ]).exec();

#制作这个

             [
                
                    "_id": "60703f09961728430c9656d1",
                    "count": 9
                ,
                
                    "_id": "60703f09961728430c",
                    "count": 4
                ,
                 
                    "_id": "60703f08430c9656d1",
                    "count": 13
                ,
            ]

这个“_id”：“60703f09961728430c9656d1”，是类集合中的Id，所以我想得到这个_id关联的名字

               [
                
                    "_id": "60703f09961728430c9656d1",
                     "name" : "Class One"
                    "count": 9
                ,
                
                    "_id": "60703f09961728430c",
                    "name" : "Class Two"
                    "count": 4
                ,
                 
                    "_id": "60703f08430c9656d1",
                    "name" : "Class Three"
                    "count": 13,
    
                ,
            ]

【参考方案1】：

您可以同时使用 $group 和 $lookup，这可能会解决您的问题。

const male = await models.Student.aggregate([
         $match:  gender: "Male"  ,
         $group:  _id: "$class", count:  $sum: 1   ,
         $lookup:  from: "class", localField: "_id", foreignField: "_id", as: "class"
    ]).exec()