Introduction

We are starting an adventure: we are going to build a great-outstanding-marvelous application using the Crystal language!

But, since these are our first steps using the Crystal language, maybe we could start with a first simple app? 🤔 Yeah! It sounds good! So, this is the goal for this post: 3 steps for creating a simple app using Crystal! 🚀

1, 2, 3 … Hold it! We are assuming that you already have installed the Crystal language. But, what if you didn't? 😱

Well, in that case here it is step 0: 

• Go to the Crystal language site and click on Install. There you'll find all you need to install the language compiler for your platform ... don't worry, we'll wait for you ...

and another step 0 😝:

• In the overview section, you will find some great examples if you didn't use Crystal before.

⏰a few minutes later

Are you ready? Great! Let's start!

Step 1: init is the magic word

So, first things first: assuming that our app is called my_app (if you can think of a better name, go ahead and rename it! 🙃) then we create the application using the same Crystal's command line, like this:

$ crystal init app my_app

And Crystal will reply:

create  my_app/.gitignore
   create  my_app/.editorconfig
   create  my_app/LICENSE
   create  my_app/README.md
   create  my_app/.travis.yml
   create  my_app/shard.yml
   create  my_app/src/my_app.cr
   create  my_app/spec/spec_helper.cr
   create  my_app/spec/my_app_spec.cr
Initialized empty Git repository in /great_projects/my_app/.git/

Perfect! We have the project with the Git repository already initialized and with a basic .gitignore (thanks Crystal! ❤️)

In the folder my_app/src/my_app.cr we have the entry point to our application. Although we may use other file, it is a standing point to start building our app.

Step 2: Hello world database!

Not the Hello World example! With step 1 we have a new Crystal App ready to grow! So let's continue with a more interesting example ... maybe we could connect to a database? Yeah! Let's do that!

We'll be using a MySQL database, It won't be necessary to install the database in our computer, instead we could use a Docker container! Here's the docker-compose.yml file:

version: '3'

services:
  db:
    image: mysql:8.0.17
    command: --default-authentication-plugin=mysql_native_password
    restart: always
    environment:
      MYSQL_ROOT_PASSWORD: root
    container_name: crystal_db
    ports:
      - 3306:3306

volumes:
  db:

Great! With this, we may start the container and have a MySQL database ready for us to use!

Let's start the container:

$ docker-compose up

If it's all good, then the response from docker should end with something like this:

crystal_db | Socket: '/var/run/mysqld/mysqlx.sock' bind-address: '::' port: 33060

Now, using our favourite database tool (in my case I'm using TablePlus), we may connect to the database using this configuration:

host: 127.0.0.1
port: 3306

user: root
password: root

And last (but not least) we will create a new database for our project called crystal123_db

CREATE DATABASE `crystal123_db`;
USE `crystal123_db`;

Step 3: (Actually) using the database from our app!

And here we are, the final step! We will need the corresponding MySQL-connector in order to use the database from our Crystal app.

First, we need to add the mysql-shard in our application's shard.yml file, like this:

dependencies:
  mysql:
    github: crystal-lang/crystal-mysql

And then, we install all the dependencies with:

$ shards install

Shards will reply:

Fetching https://github.com/crystal-lang/crystal-mysql.git
Fetching https://github.com/crystal-lang/crystal-db.git
Installing mysql (0.10.0)
Installing db (0.8.0)

Note: Shards is the Dependency manager for the Crystal language.

Now, let's try to connect to the database! (remember we called the database: crystal123_db ... a great name if you ask me 😎) And, since we are there, we will create a new table called bank_accounts (with to columns: name and amount)

# my_app.cr
require "mysql"

db = DB.open "mysql://root:root@localhost/crystal123_db"

db.exec "drop table if exists bank_accounts"
db.exec "create table bank_accounts (name varchar(30), amount int)"

Let's run the code using Crystal's command line:

$ crystal ./src/my_app.cr

Go to the database ... yes! The table bank_accounts was created!!

And for the same price, let's add 2 rows to the table. Here is the final version of my_app.cr:

# my_app.cr

require "mysql"

db = DB.open "mysql://root:root@localhost/crystal123_db"

db.exec "drop table if exists bank_accounts"
db.exec "create table bank_accounts (name varchar(30), amount int)"

# first row
db.exec "insert into bank_accounts values (?, ?)", "John", 100

# second row
db.exec "insert into bank_accounts values (?, ?)", "Sarah", 250

We run the application:

$ crystal ./src/my_app.cr

Let's check the database and ... yes! Now we have the table bank_accounts created and with 2 rows! We did it! 🎉

Farewell and see you later

We have reached the end of this first journey. To recap:

• we have initialized a Crystal application using the command line.
• we have installed a database (actually a container with the database)
• and finally we have use the database from our Crystal application!

Hope you enjoyed it! Until the next Crystal adventure!😃

References:

• Using the Crystal compiler
• Using a database with Crystal.
• The shards command