P5.3: MongoDB & Mongoose

An in-memory array will lose all its data every time the server restarts. To persist our data permanently, we must connect our API server to a database.

In this lesson, we will use MongoDB, a popular NoSQL document-based database, alongside Mongoose, an ODM (Object Data Modeling) library for Node.js.

1. Relational (SQL) vs Document (NoSQL) Databases

• SQL Databases (MySQL, PostgreSQL): Store data in strict rows and columns (tables).
• NoSQL Databases (MongoDB): Store data in flexible, JSON-like documents.

// Example of a MongoDB document
{
  "_id": "60c72b2f9b1d8b2a3c8e4d5f",
  "title": "Learn Express",
  "isCompleted": false,
  "createdAt": "2026-07-07T12:00:00Z"
}

2. Setting Up Mongoose

Mongoose provides a schema-based solution to model your application data. It includes built-in type casting, validation, and query building.

Step 1: Install Mongoose

npm install mongoose

Step 2: Connect to MongoDB Atlas

const mongoose = require('mongoose');

// Connect to local or cloud MongoDB instance
mongoose.connect('mongodb://localhost:27017/nextsem_db')
  .then(() => console.log('Successfully connected to MongoDB!'))
  .catch(err => console.error('Database connection error:', err));

3. Defining Schemas and Models

A Schema defines the structure of the document (its fields, types, and validations). A Model is a class constructed from the Schema that handles database queries.

const mongoose = require('mongoose');

// 1. Define the Schema
const TaskSchema = new mongoose.Schema({
  title: { type: String, required: true },
  isCompleted: { type: Boolean, default: false },
  createdAt: { type: Date, default: Date.now }
});

// 2. Create the Model
const Task = mongoose.model('Task', TaskSchema);

module.exports = Task;

4. Querying Data with Mongoose

Once your model is declared, you can use Mongoose methods (which return Promises) to query your database inside Express routes:

Get All Tasks:

app.get('/api/tasks', async (req, res) => {
  try {
    const tasks = await Task.find();
    res.json(tasks);
  } catch (err) {
    res.status(500).json({ error: 'Failed to retrieve tasks.' });
  }
});

Create a New Task:

app.post('/api/tasks', async (req, res) => {
  try {
    const newTask = new Task({ title: req.body.title });
    await newTask.save(); // Save to database
    res.status(201).json(newTask);
  } catch (err) {
    res.status(400).json({ error: 'Invalid data format.' });
  }
});