Docker Compose: Rails won't respect the DATABASE_URL to connect to Postgres

I also had a similar issue, the causes of which were multi-fold.

TL;DR, make sure that:

1. Your app container is correctly linked to and dependent on your postgres container
2. Your DATABASE_URL references the correct hostname of your postgres container
3. The RAILS_ENV ENV variable is explicitly set within your app container

First off, I had to make sure my app container in docker_compose.yml was linked to and depended on the postgres container:

version: '3'
    services:
      app:
        container_name: 'app'
        depends_on:
          - postgres
        links:
          - postgres

Second, the DATABASE_URL ENV variable I was providing database.yml did not reference the correct hostname of the postgres container. Originally, mine looked like:

'postgres://postgres:@localhost:5432/my_app_development'

I changed it to:

'postgres://postgres:@postgres:5432/my_app_development'

Not an apparent issue in your case, but worth noting for others.

Third, I wasn't explicitly specifying the RAILS_ENV environment. I set that ENV variable to development in docker_compose.yml so that the correct database.yml configuration was used.

The end result of my database.yml file:

development: &default
  adapter: postgresql
  encoding: unicode
  database: my_app_development
  username: postgres
  pool: 5
  timeout: 5000
  url: <%= ENV['DATABASE_URL'] %>

test:
  <<: *default
  database: my_app_test
  pool: 5
  url: <%= ENV['DATABASE_URL'] %>

production:
  <<: *default
  url: <%= ENV['DATABASE_URL'] %>

The end result of my docker_compose.yml file:

version: '3'
services:
  app:
    container_name: 'app'
    environment:
      DATABASE_URL: 'postgres://postgres:@postgres:5432/my_app_development'
      RAILS_ENV: 'development'
    build: .
    ports:
      - '3000:3000'
    volumes:
      - ./:/dev
    depends_on:
      - postgres
    links:
      - postgres
    tty: true

  postgres:
    container_name: 'postgres'
    image: postgres:9.5
    ports:
      - '5432:5432'
    environment:
      POSTGRES_USER: postgres
      POSTGRES_PASSWORD:

After that, all bundle exec rake db: commands worked just fine.

Hope that helps!

The problem is in the rake task, it is trying to create the test database, I assume your database.yml has a test environment with localhost host.

From the description of rake db:create

Creates the database from DATABASE_URL or config/database.yml for the current RAILS_ENV (use db:create:all to create all databases in the config). Without RAILS_ENV it defaults to creating the development and test databases

You should pass the env evidently or it could create a test database by default.

I suggest you to change the test database credentionals in the database.yml.

Connection Preference

I had the same issue, it seems like the config merging is somehow broken on Rails 5.1.x

You can force the use of DATABASE_URL by adding url: <%= ENV['DATABASE_URL'] %> in your default block.

I think @Зелёныйenter's answer is mostly correct, the problem is that the rake task is trying to create databases for development and test.

The line

Database 'myapp_development' already exists

is probably there because the DB was created in a previous execution of the same command, that created the DB and also failed in trying to create the test DB.

One would think that by specifying the DB name in the URL, in this case myapp_development

DATABASE_URL: postgres://postgres:pw@db:5432/myapp_development

that would be the only DB to create, but it is not. I used DATABASE_URL without specifying the DB name, like:

DATABASE_URL: postgres://postgres:pw@db:5432

with the same result, one created, one failed.

As I understand, however, this is not a problem of RAILS_ENV definition. This is expected behaviour, according to this comment on an issue about this very subject, which by the way, is still in discussion.

So far, there are 3 suggested workarounds: two are gems, and the other is the modification or elimination of database.yml.

One possible database.yml that works is:

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

development:
  <<: *default
  database: app_development

test:
  <<: *default
  database: app_test

Which will create both databases without error.