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page_title: Quickstart Guide: Compose and Rails page_description: Getting started with Docker Compose and Rails page_keywords: documentation, docs, docker, compose, orchestration, containers, rails

Getting started with Compose and Rails

This Quickstart guide will show you how to use Compose to set up and run a Rails/PostgreSQL app. Before starting, you'll need to have Compose installed.

Define the project

Start by setting up the three files you'll need to build the app. First, since your app is going to run inside a Docker container containing all of its dependencies, you'll need to define exactly what needs to be included in the container. This is done using a file called Dockerfile. To begin with, the Dockerfile consists of:

FROM ruby:2.2.0
RUN apt-get update -qq && apt-get install -y build-essential libpq-dev
RUN mkdir /myapp
WORKDIR /myapp
ADD Gemfile /myapp/Gemfile
RUN bundle install
ADD . /myapp

That'll put your application code inside an image that will build a container with Ruby, Bundler and all your dependencies inside it. For more information on how to write Dockerfiles, see the Docker user guide and the Dockerfile reference.

Next, create a bootstrap Gemfile which just loads Rails. It'll be overwritten in a moment by rails new.

source 'https://rubygems.org'
gem 'rails', '4.2.0'

Finally, docker-compose.yml is where the magic happens. This file describes the services that comprise your app (a database and a web app), how to get each one's Docker image (the database just runs on a pre-made PostgreSQL image, and the web app is built from the current directory), and the configuration needed to link them together and expose the web app's port.

version: 1.0
services:
  db:
    image: postgres
    ports:
      - "5432"
  web:
    build: .
    command: bundle exec rails s -p 3000 -b '0.0.0.0'
    volumes:
      - .:/myapp
    ports:
      - "3000:3000"
    links:
      - db

Build the project

With those three files in place, you can now generate the Rails skeleton app using docker-compose run:

$ docker-compose run web rails new . --force --database=postgresql --skip-bundle

First, Compose will build the image for the web service using the Dockerfile. Then it'll run rails new inside a new container, using that image. Once it's done, you should have generated a fresh app:

$ ls
Dockerfile   app          docker-compose.yml      tmp
Gemfile      bin          lib          vendor
Gemfile.lock config       log
README.rdoc  config.ru    public
Rakefile     db           test

Uncomment the line in your new Gemfile which loads therubyracer, so you've got a Javascript runtime:

gem 'therubyracer', platforms: :ruby

Now that you've got a new Gemfile, you need to build the image again. (This, and changes to the Dockerfile itself, should be the only times you'll need to rebuild.)

$ docker-compose build

Connect the database

The app is now bootable, but you're not quite there yet. By default, Rails expects a database to be running on localhost - so you need to point it at the db container instead. You also need to change the database and username to align with the defaults set by the postgres image.

Open up your newly-generated database.yml file. Replace its contents with the following:

development: &default
  adapter: postgresql
  encoding: unicode
  database: postgres
  pool: 5
  username: postgres
  password:
  host: db

test:
  <<: *default
  database: myapp_test

You can now boot the app with:

$ docker-compose up

If all's well, you should see some PostgreSQL output, and then—after a few seconds—the familiar refrain:

myapp_web_1 | [2014-01-17 17:16:29] INFO  WEBrick 1.3.1
myapp_web_1 | [2014-01-17 17:16:29] INFO  ruby 2.2.0 (2014-12-25) [x86_64-linux-gnu]
myapp_web_1 | [2014-01-17 17:16:29] INFO  WEBrick::HTTPServer#start: pid=1 port=3000

Finally, you need to create the database. In another terminal, run:

$ docker-compose run web rake db:create

That's it. Your app should now be running on port 3000 on your Docker daemon (if you're using Boot2docker, boot2docker ip will tell you its address).

More Compose documentation