Introduce Docker to the development environment and test environment of existing Rails and MySQL applications


As the title suggests, Docker will be installed in the existing Rails and MySQL application development environment. It also allows you to run rspec tests within Docker. There are already a lot of similar articles, but I've encountered quite a few errors, so I'll summarize how I worked. I would appreciate it if you could let me know if there are any mistakes.


If Rails version is 6 and MySQL version is 8, this method will not work.


  1. Create Dockerfile
  2. Create docker-compose.yml
  3. Modify the existing database.yml
  4. Start up

1. Create Dockerfile

Create a new Dockerfile in the existing project directory.

file organization

 test_app#Existing project
├── Abbreviation
    ├── Dockerfile #Create
    ├── docker-compose.yml #Create
    ├── Gemfile
    ├── Gemfile.lock
    ├── .env #use
    ├── config
         └──database.yml #Change

I will describe the contents. Please match test_app below to each app name.


FROM ruby:2.6.5

RUN apt-get update \
    && apt-get install -y --no-install-recommends nodejs mariadb-client build-essential \
    && apt-get clean && rm -rf /var/lib/apt/lists/*

WORKDIR /test_app

COPY Gemfile /test_app/Gemfile
COPY Gemfile.lock /test_app/Gemfile.lock

RUN gem install bundler
RUN bundle install
COPY . /test_app

--First, get the image of ruby with FROM. The version suits your environment. --Next, you are installing the required packages. There is an article that mariadb-client is mysql-client, but note that it resulted in an error. mysql-client seems to be integrated with mariadb-client. --Next, specify the working directory. Even if there is no RUN mdir, it will be created with WORKDIR. --The following is copying gemfile etc. from local to the container. There is an article that COPY is ʻADD, but it seems that COPY` is recommended.

2. Create docker-compose.yml


version: '3'

    image: mysql:5.7
    command: mysqld --character-set-server=utf8 --collation-server=utf8_unicode_ci
      - '3306:3306'
      MYSQL_DATABASE: today_code_development
      - mysql-data:/var/lib/mysql

    image: mysql:5.7
    command: mysqld --character-set-server=utf8 --collation-server=utf8_unicode_ci
      MYSQL_DATABASE: today_code_test
      - '3307:3306'

      context: .
      dockerfile: Dockerfile
    command: bundle exec rails s -p 3000 -b ''
    tty: true 
    stdin_open: true
      - db
      - test-db
      - "3000:3000" 
      - .:/today_code
      - bundle:/usr/local/bundle


Write only the points.

--The command line sets the character code of database to utf8. Without this, an error occurred at db: create. --In ʻenviroment, the user name and password are set. Of course, it works even if you write it directly, but here we are expanding the environment variables. The docker-compose file seems to read the .env file by default, so define it in the .env file as shown below. The defined environment variables can be expanded with $ {}`. Add the .env file to the .gitignore file so that it doesn't commit. --The data is persisted by creating named volumes with volumes. There seems to be a way to mount the directory, but the data volume seems to have more advantages.



3. Modify database.yml


default: &default
  adapter: mysql2
  encoding: utf8
  charset: utf8
  pool: 5
  port: 3306

  <<: *default
  database: test_app_development
  username: <%= ENV['DATABASE_DEV_USER'] %>
  password: <%= ENV['DATABASE_DEV_PASSWORD'] %>
  host : db

  <<: *default
  database: test_app_test
  username: <%= ENV['DATABASE_DEV_USER'] %>
  password: <%= ENV['DATABASE_DEV_PASSWORD'] %>
  host : test-db

  <<: *default
  database: <%= ENV['DB_NAME'] %>
  username: <%= ENV['DB_USERNAME'] %>
  password: <%= ENV['DB_PASSWORD'] %>
  host: <%= ENV['DB_HOSTNAME'] %>

--username and password are the same as docker-compose. --It is set to connect to the db container created by host. ――Please adjust the production to each environment --The dotenv-rails gem is used to manage environment variables.

4. Start up

If you can start docker at the end, it is successful.

  1. Create an image with docker-compose build
  2. Launch the container with docker-compose up -d. -d is an option to start in the background.
  3. Create database with docker-compose run web rails db: create

If it is displayed at http: // localhost: 3000 /, it is successful. All you have to do is db: migrate or db: seed and you're done.

Reference article

I tried to build an existing Rails project development environment using Docker (Rails + Mysql) How to run an existing Rails application with a Docker container + DB container visualization with sequence pro Build a test DB in Rails x Docker environment Persist Docker data! Introduction to environment construction starting from understanding Data Volume

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