Mongodb$lookup无法使用_ID

Question

wend尝试此查询，返回查找为空

db.getCollection('tests').aggregate([
    {$match: {typet:'Req'}},
    {$project: {incharge:1}},
    {$lookup:{
            from: "users",
            localField: "incharge", //this is the _id user from tests
            foreignField: "_id", //this is the _id from users
            as: "user"
    }}
])

返回json

  [
    {
        "_id": "57565d2e45bd27b012fc4db9",
        "incharge": "549e0bb67371ecc804ad23ef",
        "user": []
    },
    {
        "_id": "57565d2045bd27b012fc4cbb",
        "incharge": "549e0bb67371ecc804ad21ef",
        "user": []
    },
    {
        "_id": "57565d2245bd27b012fc4cc7",
        "incharge": "549e0bb67371ecc804ad24ef",
        "user": []
    }
]

我尝试了这篇文章，但没有发生MongoDB将项目字符串聚合到ObjectId和这个MongoDB$lookup在PHP中使用_id作为foreignField

更新

这是文档“用户”

    {
        "_id" : ObjectId("549e0bb67371ecc804ad24ef"),
        "displayname" : "Jhon S."
    },
    {
        "_id" : ObjectId("549e0bb67371ecc804ad21ef"),
        "displayname" : "George F."
    },
    {
        "_id" : ObjectId("549e0bb67371ecc804ad23ef"),
        "displayname" : "Franc D."
    }

Answer 1

首先，断言incharge字段的类型为mongoose.schema.types.objectid。如果仍然返回空数组，可能是因为您使用了在NodeJS中声明的模式名，而不是MongoDB使用的集合名。

userschema文件中的示例：

const mongoose = require('mongoose')
const Schema = mongoose.Schema

const UserSchema = new Schema({
  name: { 
    type: String, 
    required: true
  },
  incharge: { 
    type: Schema.Types.ObjectId, 
    required: true
  },
})

const User = mongoose.model('User', UserSchema)
module.exports = User

上面的模型由mongoose命名为user，但mongoDB中的相应集合命名为users。结果的$lookup写成：

$lookup:{
  from: "users",           // name of mongoDB collection, NOT mongoose model
  localField: "incharge",  // referenced users _id in the tests collection
  foreignField: "_id",     // _id from users
  as: "user"               // output array in returned object
}

https://mongoosejs.com/docs/models.html
https://mongoosejs.com/docs/schematypes.html

Answer 2

我最终找到了解决方案，是我在mongoose中使用ObjectId的模式出现了问题

我换了这个

var Schema = new Schema({
    name: { type: String, required: true},
    incharge: { type: String, required: true},
});

用这个

var Schema = new Schema({
    name: { type: String, required: true},
    incharge: { type: mongoose.Schema.ObjectId, required: true},
});

正在工作