Lesson 5 - Introduction to MongoDB

In the previous lesson, Complete RESTful API in Node.js, we implemented a complete RESTful API with Express. But we stored data only in an array, so we lost it every time we restarted the application. Therefore, it'd be useful to have a database in the project where the data would be stored permanently.

Choosing the Database

We can choose from many available databases. On the one hand, we have relational databases (MySQL, MS-SQL, and other) that use tables and we query data using SQL statements. However, non-relational or document-oriented databases are more popular for Node and Express projects. The best known is MongoDB.

MongoDB

MongoDB is a NoSQL database. It's object-oriented, simple, dynamic and well scalable. It uses collections instead of tables (as in a relational database), instead of rows and columns it uses documents and fields. The data is stored in the JSON format, which is useful because we already have our data in the JSON format, and we won't have to convert it anyhow. (More precisely, they are stored on the disk as BSON, which is a binary JSON - but this is not very important.) The data don't have any predefined format (MongoDB is said to be schema-less). Objects can be nested in each other, so what you'd have to do with multiple linked tables in a relational database can be done in a single document in MongoDB.

Installation

On the https://www.mongodb.com page, click Try Free at the top right and select the server tab. You should have the MongoDB Community Server selected. Next, select the version (or leave the selected one) and select your operating system. Leave the package as MSI and download the installer. Select the complete installation type. Somewhen during the installation, the installer will ask if you want to install MongoDB Compass. You definitely want it, but sometimes it can be a problem to install it together with the database - if that happens, install the database without it and then install it separately. It can be found at the same page in the Tools tab.

The MongoDB homepage often changes, so everything may not match exactly, but downloading should always be easy. To begin with, you want the MongoDB Community Server and to install MongoDB Compass as well.

Now, move to the C:\Program Files\MongoDB\Server\4.0\bin folder in the command line (customize the version according to the current version of your installation) and run MongoDB with the mongod command:

cd C:\Program Files\MongoDB\Server\4.0\bin
mongod

To make it easier to use, it pays off to add the above path to the PATH variable in environment variables. The steps varies according to your Windows version, in Windows 10 it's in Control Panel -> System and Security -> System -> View advanced system settings. Next, click on the environment variables, find the PATH variable, click Edit ..., click New, and add the path to Mongo. Then just click Ok three times. You should now be able to run the mongodb command from anywhere. (If you set it up correctly and it still doesn't work, try restarting the command line.)

When you start Mongo, it'll print quite a lot of things, and if you've never had Mongo on your computer before, it'll probably throw an exception. It'll be about four lines before the end of the output and will contain something like NonExistentPath: Data directory C:\data\db\ not found., terminating. This is because Mongo stores its data in the C:\data\db directory by default. Let's create it:

md c:\data\db

Run mongo again, it should run fine now, and the last message should be that it's waiting on the port 27017.

Run installed Compass, click yourself through "Next" and "Get Started", leave the default settings (i.e. localhost and the port 27017) on the connection page and connect. You should see the default Mongo databases (certainly admin and local, possibly config).

Mac Installation Notes

On Mac, you can simplify the installation with Homebrew. Homebrew is a package manager (something like npm) for macOS and Linux. Follow the instructions at https://brew.sh/ to install it. Now you can install Mongo with a simple brew install mongodb command. Be sure to create the database directory and to install Compass.

Alternative Solution

If you don't want, you don't need to download Mongo at all. There's also DBaaS (database as a service), where you can use a platform to offer you a hosted database.

We can use MLab (mlab.com) with various packages available, the trial package (which is enough for our first projects) is free.

Also, Mongo now offers the Atlas cloud service, where you can also host your databases for free. It can be found directly on the Mongo main page. If you try it, you can write your experiences in the comments, and I'm curious about them.

Connecting to the Database

Let's start a new project. We'll install the mongoose package into it. Thanks to mongoose we'll be able to work with Mongo through a simple API:

mkdir mongo-project
cd mongo-project
npm init --yes
npm install mongoose

Create the index.js file and paste the following code into it:

const mongoose = require('mongoose');

mongoose.connect('mongodb://localhost:27017/moviesdb', { useNewUrlParser: true })
  .then(() => console.log('Connected to MongoDB!'))
  .catch(error => console.error('Could not connect to MongoDB... ', error));

First, we load the mongoose module and store it as a constant containing an object. This object has the connect() method that allows us to connect to the database. It takes the connection string as the first parameter. We hard-coded it for now. In a real application, you'll rather read the connection string from the configuration file. This will be mongodb://localhost:27017/moviesdb, where moviesdb is the database name.

The fact that our database doesn't yet exist doesn't matter at all. MongoDB creates it the first time we connect to it.

The second parameter is only configuration for newer Mongo versions (> = 3.1.0).

Because the connect() method returns a promise (see the note at the end of the article), we can call then() on it to react on a successful call and catch() to react to an error.

We now just run the project (node index.js), and we should see a successful connection message if everything went OK.

Next time, in the lesson First steps in MongoDB, we'll talk a little bit more about mongoose and show you how to write and read data using it.

Final note: Promises

Since ES6 promises are often use to handle asynchronous calls in JavaScript. Promise is a promise of some future value. Connecting to a database is an example of an asynchronous operation (it takes a while to connect, so we don't know the result right away, and we must wait for it - but thanks to asynchronicity, other code can normally run while waiting). Promise is one way to deal with this (another would be the async/await syntax which we'll use next time).

Promise can be in three states - pending, fulfilled and rejected. It also has the then() method which takes a function as a parameter - it's called if the promise succeeds. There's also the catch() method, whose parameter (also a function) is called in case of rejection, i.e. of an error.