Rails with Docker-compose: how to set up database host

Question

One thing that I can't figure out is how to set up a database host in case when we dockerize a Rail app ? For example, a Postgres DB is supposed to run on localhost on a dev machine. But in a the docker-compose file the database service has its own name, - it's on that host that the database will be accessible for other containers, foe example:

version: '3'

services:
  web:
    build: .
    ports:
      - "3000:3000"
    volumes:
      - .:/usr/src/app
    env_file:
      - .env/development/database
      - .env/development/web
  redis:
    image: redis
  database:
    image: postgres
    env_file:
      - .env/development/database
    volumes:
      - db-data:/var/lib/postgresql/data
volumes:
  db-data:

Most examples suppose to execute all the commands related the development Rails (creating models, migrations, etc.) from inside the container, e.g.

docker-compose exec web rails g scaffold User first_name:string last_name:string

And to run the above migration I'd have to run

docker-compose exec web rails db:migrate

This way it works. But why do I need to run Docker for my dev locally to be able to access the app ?

So I come back to my original essential question: when the app was generated, database.yml had the below settings (for Postgres):

default: &default
  adapter: postgresql
  encoding: unicode
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>

development:
  <<: *default
  database: rails5-ember_development

This way, everybody could clone the project and continue to develop when having Postgres DB running on localhost. Now when dockerizing the app, how to change/adapt the host value, - localhost:5432 being by default so that the application could run both in a Docker container ?

So, to resume my question is:

To be able to simulate the same behaviour in a dockerized Rails app, is the only solution would be to run it in a special environment other than development? In this case, I'd add it to database.yml and set the same DB values as in docker-compose.yml file (username, host, etc.).

Thank you.

Can you please modulate your questions? It's a bit difficult to understand what exactly the problem is! — Janshair Khan
– Janshair Khan, Commented Jun 18, 2018 at 15:05
I've updated the question, see the last paragraph. Thank you. — belgoros
– belgoros, Commented Jun 19, 2018 at 7:07
@emix: I'd like to be able: to dockerize a Rails app and have it to run the same way: in a Docker container and locally, i.e. to be able to develop the app in a classic way without a need to run Rails generators command without prefixing them with docker-compose exec .... — belgoros
– belgoros, Commented Jun 19, 2018 at 7:25
You can tell docker-compose to use specific entrypoint in which you would instantiate your container, if I understand correctly. You can do that in a separate docker-compose file. — Mike Doe
– Mike Doe, Commented Jun 19, 2018 at 7:26

belgoros · Accepted Answer · 2018-06-20 15:06:29Z

Here is the solution I came to.

Create a Dockerfile as follows in the Rails app:

F

FROM ruby:2.5.1
LABEL maintainer="Serguei CAMBOUR <[email protected]>"
RUN apt-get update -yqq
RUN apt-get install -yqq --no-install-recommends nodejs
COPY Gemfile* /usr/src/app/
WORKDIR /usr/src/app
RUN bundle install
COPY . /usr/src/app/
CMD ["rails", "s", "-b", "0.0.0.0"]

Create docker-compose.yml as follows:

v

version: '3'

services:
  web:
    build: .
    ports:
      - "3000:3000"
    volumes:
      - .:/usr/src/app
    env_file:
      - .env/development/database
      - .env/development/web
  redis:
    image: redis
  database:
    image: postgres
    env_file:
      - .env/development/database
    volumes:
      - db-data:/var/lib/postgresql/data
volumes:
  db-data:

Create .env/development/database file as follows:

P

POSTGRES_USER=postgres
POSTGRES_DB=myapp_development

Create .env/development/web file as follows:

DATABASE_HOST=database

Change the settings in database.yml as follows to be able to read env variable values:

d

default: &default
  adapter: postgresql
  encoding: unicode
  host: <%= ENV['DATABASE_HOST'] %>
  username: <%= ENV['POSTGRES_USER'] %>
  database: <%= ENV['POSTGRES_DB'] %>
  pool: 5
  variables:
    statement_timeout: 5000

development:
  <<: *default

test:
  <<: *default
  database: myapp_test

production:
  <<: *default

Now, you can run your rails app as usual with rails s and it will work. The same is for running all the Rails generators, migrations etc, - it will work and communicate with your Postgresql DB in local.

To run your code in a docker container:

Run docker-compose build web (where web is the name of my service declared in docker-compose.yml before).
Or just start and build it with docker-compose up --build.
Navigate to one of your routes to check if it works.
Create the database if it does not exist yet: docker-compose run --rm web rails db:create.
Run some pending migrations if there are some: docker-compose exec web rails db:migrate.
You can combine the both: docker-compose exec web rails db:create db:migrate.

Hope this helps.

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