Connecting an Express.js Application to MongoDB and PostgreSQL: A Step-by-Step Guide for Beginners

📖 Table of Contents

1. Introduction to Databases
2. MongoDB vs PostgreSQL: Key Differences
3. Setting Up MongoDB
4. Connecting MongoDB to Express.js
5. Setting Up PostgreSQL
6. Connecting PostgreSQL to Express.js
7. Implementing CRUD Operations with a Database
8. Environment Variables for Database Configuration
9. Best Practices
10. Conclusion

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1. Introduction to Databases

Databases are essential for any web application to store, retrieve, and manage data efficiently.

• MongoDB: A NoSQL database that stores data in JSON-like documents. Great for unstructured data.
• PostgreSQL: A relational SQL database ideal for structured data with complex relationships.

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2. MongoDB vs PostgreSQL: Key Differences

Feature	MongoDB	PostgreSQL
Type	NoSQL	SQL
Schema	Flexible	Fixed Schema
Scalability	Horizontal	Vertical
Best for	Real-time apps, Document storage	Complex queries, Structured data

Choose MongoDB for flexibility and PostgreSQL for structured and relational data.

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3. Setting Up MongoDB

3.1 Install MongoDB Locally

• Download MongoDB from the official MongoDB website.
• Install MongoDB Compass (GUI tool for MongoDB).

3.2 Set Up MongoDB Connection URL

MONGODB_URI=mongodb://localhost:27017/express_crud_db

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4. Connecting MongoDB to Express.js

4.1 Install MongoDB Dependencies

npm install mongoose

4.2 Create Database Configuration File

File: src/config/database.ts

import mongoose from 'mongoose';

const connectMongoDB = async () => {
  try {
    await mongoose.connect(process.env.MONGODB_URI || '');
    console.log('MongoDB Connected');
  } catch (error) {
    console.error('MongoDB connection failed:', error);
    process.exit(1);
  }
};

export default connectMongoDB;

4.3 Update app.ts

import connectMongoDB from './config/database';
connectMongoDB();

4.4 Create a User Model

File: src/models/userModel.ts

import mongoose, { Schema, Document } from 'mongoose';

interface IUser extends Document {
  name: string;
  email: string;
}

const UserSchema: Schema = new Schema({
  name: { type: String, required: true },
  email: { type: String, required: true }
});

export default mongoose.model<IUser>('User', UserSchema);

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5. Setting Up PostgreSQL

5.1 Install PostgreSQL Locally

• Download PostgreSQL from the official website.
• Install pgAdmin (GUI tool for PostgreSQL).

5.2 Create a Database

• Open psql or pgAdmin.
• Run:

CREATE DATABASE express_crud_db;

5.3 Set Up PostgreSQL Connection URL

POSTGRESQL_URI=postgresql://user:password@localhost:5432/express_crud_db

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6. Connecting PostgreSQL to Express.js

6.1 Install PostgreSQL Dependencies

npm install knex pg

6.2 Configure Knex.js

File: src/config/knex.ts

import knex from 'knex';

const db = knex({
  client: 'pg',
  connection: process.env.POSTGRESQL_URI,
  pool: { min: 0, max: 7 }
});

export default db;

6.3 Create a Migration File

npx knex migrate:make create_users_table --knexfile src/config/knex.ts

File: src/migrations/<timestamp>_create_users_table.ts

export function up(knex) {
  return knex.schema.createTable('users', (table) => {
    table.increments('id').primary();
    table.string('name').notNullable();
    table.string('email').notNullable().unique();
  });
}

export function down(knex) {
  return knex.schema.dropTable('users');
}

Run the migration:

npx knex migrate:latest --knexfile src/config/knex.ts

6.4 Create a User Model

File: src/models/userModel.ts

import db from '../config/knex';

export const getAllUsers = () => db('users').select('*');
export const getUserById = (id: number) => db('users').where({ id }).first();
export const createUser = (user: { name: string; email: string }) => db('users').insert(user);
export const updateUser = (id: number, user: { name?: string; email?: string }) => db('users').where({ id }).update(user);
export const deleteUser = (id: number) => db('users').where({ id }).del();

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7. Implementing CRUD Operations with Database

7.1 Update Controller for MongoDB

File: src/controllers/userController.ts

import User from '../models/userModel';

export const getUsers = async (req, res) => {
  const users = await User.find();
  res.json(users);
};

7.2 Update Controller for PostgreSQL

import { getAllUsers, getUserById, createUser, updateUser, deleteUser } from '../models/userModel';

export const getUsers = async (req, res) => {
  const users = await getAllUsers();
  res.json(users);
};

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8. Environment Variables for Database Configuration

File: .env

MONGODB_URI=mongodb://localhost:27017/express_crud_db
POSTGRESQL_URI=postgresql://user:password@localhost:5432/express_crud_db

Update your app.ts to load .env:

import dotenv from 'dotenv';
dotenv.config();

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9. Best Practices

1. Use environment variables for sensitive data.
2. Validate data before saving to the database.
3. Use connection pools for PostgreSQL.
4. Properly handle database connection errors.
5. Avoid exposing raw database errors to the client.

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10. Conclusion

• MongoDB: Great for dynamic, schema-less applications.
• PostgreSQL: Best for structured, relational data with strict validation.

Choose the database based on your project requirements.

✨ Your Express.js app is now connected to both MongoDB and PostgreSQL! 🚀

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Author: Mohin Sheikh

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