Population for references in Express - Mini Project: Build & Apply

Population for references

📖 Scenario: You are building a simple Express.js server that manages a list of books and their authors. Each book stores only the author's ID. You want to show the full author details when you fetch the books by using population for references.

🎯 Goal: Create an Express.js app that defines two Mongoose models: Author and Book. Store authors and books separately. Then, write a route to fetch all books with their author details populated.

📋 What You'll Learn

Create a Mongoose model called Author with fields name and age.

Create a Mongoose model called Book with fields title and author (reference to Author).

Add a config variable authorId with a sample ObjectId string.

Write a route GET /books that fetches all books and populates the author field.

Use populate('author') to fill author details in the book documents.

💡 Why This Matters

🌍 Real World

Population for references is used in real apps to link related data stored in different collections, like users and their posts, or products and their categories.

💼 Career

Understanding how to use Mongoose population is important for backend developers working with MongoDB and Express to build APIs that return rich, connected data.

1

Define Mongoose models for Author and Book

Create a Mongoose schema and model called Author with fields name (String) and age (Number). Also create a Mongoose schema and model called Book with fields title (String) and author (ObjectId referencing Author).

Express

import express from 'express';
import mongoose from 'mongoose';

await mongoose.connect('mongodb://localhost:27017/library');

const app = express();

// Define Author schema and model
// Define Book schema and model
// Your code here

Need a hint?

Use new mongoose.Schema to define schemas. Use ref: 'Author' for the author field in Book schema.

2

Add a sample authorId variable

Create a constant variable called authorId and assign it the string value '64a7f0c2b4d1a2e5f3c9b123'.

Express

import express from 'express';
import mongoose from 'mongoose';

await mongoose.connect('mongodb://localhost:27017/library');

const app = express();

const authorSchema = new mongoose.Schema({
  name: String,
  age: Number
});
const Author = mongoose.model('Author', authorSchema);

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

// Add a constant authorId variable
// Your code here

Need a hint?

Use const authorId = '64a7f0c2b4d1a2e5f3c9b123' to create the variable.

3

Create a GET /books route with population

Write an Express route handler for GET /books that fetches all books from the database and uses populate('author') to fill in the author details.

Express

import express from 'express';
import mongoose from 'mongoose';

await mongoose.connect('mongodb://localhost:27017/library');

const app = express();

const authorSchema = new mongoose.Schema({
  name: String,
  age: Number
});
const Author = mongoose.model('Author', authorSchema);

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

const authorId = '64a7f0c2b4d1a2e5f3c9b123';

// Create GET /books route with populate
// Your code here

Need a hint?

Use Book.find().populate('author') inside the route handler to get books with author details.

4

Start the Express server

Add code to start the Express server on port 3000 using app.listen.

Express

import express from 'express';
import mongoose from 'mongoose';

await mongoose.connect('mongodb://localhost:27017/library');

const app = express();

const authorSchema = new mongoose.Schema({
  name: String,
  age: Number
});
const Author = mongoose.model('Author', authorSchema);

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

const authorId = '64a7f0c2b4d1a2e5f3c9b123';

app.get('/books', async (req, res) => {
  const books = await Book.find().populate('author');
  res.json(books);
});

// Start the server on port 3000
// Your code here

Need a hint?

Use app.listen(3000) to start the server on port 3000.