app.js

const mongoose = require('mongoose')
const {model,Schema} = require('mongoose')

mongoose.connect('mongodb://localhost:27017/myapp',{useNewUrlParser:true});
const app = require('express')()
const DepartmentSchema = new Schema({
        name: String,
        location: String
    }
)

var Department = model("department",DepartmentSchema)
var EmployeeSchema = new Schema({
    firstName: String,
    lastName: String,
    mobile: String,
    department: {type: Schema.Types.ObjectId,ref:'department'}
})
var Employee = model("employee",EmployeeSchema)
var CompanySchema = new Schema({
    name: String,
    address: String,
    employee: [{type: Schema.Types.ObjectId,ref:'employee'}]
})
var Company = model("company",CompanySchema)

app.use("/",async(req,res)=> {
    await Department.remove({})
        await Department.create({
            name: 'IT Department',
            location: 'Building A'
        })
        await Department.create({
            name: 'Marketing Department',
            location: 'Building B'
        })
    await Employee.remove({})
    await Employee.create({
        firstName: 'PYSUS',
        lastName: 'PY',
        monile: "321",
        department: await Department.findOne({name:'IT Department'})
    })
    await Employee.create({
        firstName: 'Sunder',
        lastName: 'PY',
        monile: "123",
        department: await Department.findOne({name:'Marketing Department'})
    })

    await Company.remove({})
    await Company.create({
        "name": 'BigCompany',
        "address": 'Address',
        "employee": await Employee.find()
    })
    res.json({
        departments: await Department.find(),
        employees: await Employee.find(),
        employeesWithDep: await Employee.find().populate("department","name"),
        company: await Company.find(),
        companyWithEmp: await Company.find().populate("employee"),
        companyWithEmpAndDep: await Company.find()
        .populate({
            path: "employee",
            model: "employee",
            populate: {path: "department",model:"department"}
        })

    }
    )
})

app.listen(3050,()=> console.log("Listening on port 3050"))

gistfile1.txt

{
  "name": "mongoose-relation",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "start": "nodemon app.js"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "dependencies": {
    "express": "^4.17.1",
    "mongoose": "^6.0.14"
  },
  "devDependencies": {
    "nodemon": "^2.0.15"
  }
}