How to Create Relationships with Mongoose and Node.JS?

// app.js code....

const express = require("express");
const bodyParser = require("body-parser");
const connectDB = require("./database/db");
const app = express();
const PORT = process.env.PORT || 3000;

// Connect to the database
connectDB();

app.set("view engine", "ejs");
app.use(bodyParser.urlencoded({ extended: true }));
app.use(express.static("public"));

const authorRouter = require("./routes/authors");
const bookRouter = require("./routes/books");

app.use("/authors", authorRouter);
app.use("/books", bookRouter);

app.get("/", (req, res) => {
    res.redirect("/authors");
});

app.listen(PORT, (err) => {
    if (err) {
        console.log(err);
    } else {
        console.log(`Server is running on http://localhost:${PORT}`);
    }
});

// routes/authors.js code .....

const express = require("express");
const router = express.Router();
const Author = require("../models/author");

router.get("/", async (req, res) => {
    const authors = await Author.find({});
    res.render("authors", { authors });
});

router.post("/", async (req, res) => {
    const author = new Author({
        name: req.body.name,
    });
    await author.save();
    res.redirect("/authors");
});

module.exports = router;

// routes/books.js code .....

const express = require("express");
const router = express.Router();
const Book = require("../models/book");
const Author = require("../models/author");

router.get("/", async (req, res) => {
    const books = await Book.find({}).populate("author");
    const authors = await Author.find({});
    res.render("books", { books, authors });
});

router.post("/", async (req, res) => {
    const book = new Book({
        title: req.body.title,
        author: req.body.author,
    });
    await book.save();
    res.redirect("/books");
});

module.exports = router;

// models/author.js code .....

const mongoose = require('mongoose');

const authorSchema = new mongoose.Schema({
    name: {
        type: String,
        required: true
    }
});

module.exports = mongoose.model('Author', authorSchema);

// models/book.js code .....


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

const bookSchema = new Schema({
    title: {
        type: String,
        required: true
    },
    author: {
        type: Schema.Types.ObjectId,
        ref: 'Author',
        required: true
    }
});

module.exports = mongoose.model('Book', bookSchema);

// database/db.js

const mongoose = require('mongoose');

const connectDB = async () => {
    try {
        await mongoose.connect('mongodb://localhost:27017/mongoose-relations', {
           
        });
        console.log('Database is connected');
    } catch (err) {
        console.error('Error connecting to the database:', err);
        process.exit(1);
    }
};

module.exports = connectDB;