Rails app using docker-compose wont connect to the mysql container

Question

Trying to get a rails app running using docker-compose. I run docker-compose build and it completes with no errors. I then run docker-compose up and both of the containers start. Then I run docker-compose run web rake db:create db:migrate and run into an error:

rake aborted! Mysql2::Error::ConnectionError: Can't connect to MySQL server on '127.0.0.1' (111 "Connection refused")

My Dockerfile:

FROM ruby:2.5.3
RUN apt-get update -qq && apt-get install -y build-essential libpq-dev nodejs mysql-client sqlite3 zlib1g-dev libxslt-dev git  && \
gem install bundler
RUN mkdir /myapp
WORKDIR /myapp
COPY Gemfile /myapp/Gemfile
COPY Gemfile.lock /myapp/Gemfile.lock
RUN bundle install
COPY . /myapp

Docker-compose

version: '3.3'
services:
  db:
    image: mysql:5.7
    restart: always
    environment:
      MYSQL_DATABASE: 'online_community_development'
      MYSQL_ROOT_PASSWORD: '******'
      MYSQL_HOST: 'localhost'
    ports:
      # <Port exposed> : < MySQL Port running inside container>
      - '3306:3306'
    expose:
      # Opens port 3306 on the container
      - '3306'
      # Where our data will be persisted
    volumes:
      - 'my-db:/var/lib/mysql'
    container_name: datab




  web:
    build: .
    command: bundle exec rails s -p 3000 -b '0.0.0.0'
    volumes:
    - web-app:/myapp
    ports:
    - "3000:3000"
    depends_on:
    - db
    links:
      - db:db
    restart: always

volumes:
  my-db: {}
  web-app: {}

Database.yml

development:
  adapter: mysql2
  encoding: utf8
  database: online_community_development
  pool: 5
  username: root
  password: slumland
  socket: /var/run/mysqld/mysqld.sock
  host: db

New to docker so not even sure if I am issuing the commands correctly. I need to create and seed the database and have the app container connect to it. I think the issue is related to it trying to connect via localhost, but even when I changed the host to db I still got the same results. Any help would be greatly appreciated.

Answer 1

tl;dr:
Try this docker-compose.yml:

version: '3.3'
services:
  db:
    image: mysql:5.7
    restart: always
    environment:
      MYSQL_DATABASE: 'online_community_development'
      MYSQL_ROOT_PASSWORD: '******'
      MYSQL_HOST: 'localhost'
    expose:
    # Opens port 3306 on the container
      - '3306'
    # Where our data will be persisted
    volumes:
      - 'my-db:/var/lib/mysql'
    container_name: datab

  web:
    build: .
    command: bundle exec rails s -p 3000 -b '0.0.0.0'
    environment:
      DATABASE_URL: 'mysql2://db'
      RAILS_ENV: 'development'
    volumes:
    - web-app:/myapp
    ports:
    - "3000:3000"
    depends_on:
    - db
    restart: always

volumes:
  my-db: {}
  web-app: {}

In detail:
First of all your docker-compose-file is kinda misconfigured.
Therefore I will do a small instruction on docker-compose.yml-files before heading over to your question.

The links:-thing is a legacy feature of docker which you should strictly avoid:
https://docs.docker.com/compose/compose-file/#links
Moreover, it is not even required when you're working with docker-compose

Also, you're using expose: and ports: when defining your database-service.
The ports-key is opening up a port on your host-machine and forwards it to your docker-service:
https://docs.docker.com/compose/compose-file/#ports
Using ports-key on database-definitions makes your database accessible to the www - which is not what you want in most cases.

The expose-key opens the port locally, in the created docker network.
https://docs.docker.com/compose/compose-file/#expose
This is what we want for databases.

Editing your docker-compose.yml-file results in the following:

version: '3.3'
services:
  db:
    image: mysql:5.7
    restart: always
    environment:
      MYSQL_DATABASE: 'online_community_development'
      MYSQL_ROOT_PASSWORD: '******'
      MYSQL_HOST: 'localhost'
    expose:
    # Opens port 3306 on the container
      - '3306'
    # Where our data will be persisted
    volumes:
      - 'my-db:/var/lib/mysql'
    container_name: datab

  web:
    build: .
    command: bundle exec rails s -p 3000 -b '0.0.0.0'
    volumes:
    - web-app:/myapp
    ports:
    - "3000:3000"
    depends_on:
    - db
    restart: always

volumes:
  my-db: {}
  web-app: {}

Getting back to your primary problem:
rake aborted! Mysql2::Error::ConnectionError: Can't connect to MySQL server on '127.0.0.1' (111 "Connection refused")

It looks like rake expects the database to be available at localhost - therefore I think that the cause of this issue is a misconfiguration.
Unfortunately I'm not very familiar with rake but nevertheless your database.yaml looks correct.
I did some research and came across this post:
Rake tasks seem to ignore database.yml configuration
Maybe you could try to set the environment-variable DATABASE_URL in your docker-compose-file: DATABASE_URL=mysql://db
I'd also recommend to set the correct context-environment-variable: RAILS_ENV=development

When you do alle these changes you should come up with the following docker-compose.yml:

version: '3.3'
services:
  db:
    image: mysql:5.7
    restart: always
    environment:
      MYSQL_DATABASE: 'online_community_development'
      MYSQL_ROOT_PASSWORD: '******'
      MYSQL_HOST: 'localhost'
    expose:
    # Opens port 3306 on the container
      - '3306'
    # Where our data will be persisted
    volumes:
      - 'my-db:/var/lib/mysql'
    container_name: datab

  web:
    build: .
    command: bundle exec rails s -p 3000 -b '0.0.0.0'
    environment:
      DATABASE_URL: 'mysql2://db'
      RAILS_ENV: 'development'
    volumes:
    - web-app:/myapp
    ports:
    - "3000:3000"
    depends_on:
    - db
    restart: always

volumes:
  my-db: {}
  web-app: {}

Hope this helps.

Regards,
Hermsi