Getting "FATAL: role "root" does not exist" when trying to do "docker up"

Question

I'm starting to work with an existing Rails project that uses Docker. I've used Rails for a long time but never Docker.

After I do a docker build . I try to do a docker-compose up, but I get:

FATAL: role "root" does not exist /usr/local/bundle/gems/activerecord-4.2.5.2/lib/active_record/connection_adapters/postgresql_adapter.rb:661:in `rescue in connect': FATAL: role "root" does not exist (ActiveRecord::NoDatabaseError)

It seems to me that the Docker machine is probably trying to connect to the database as the root user, but there's no role called root, so the connection is rightly failing.

The thing I don't know is why Docker is apparently trying to connect to the database as root and how to get it to use the right user.

Here's my database.yml:

development:
  database: my_app_development
  adapter: postgresql
  encoding: unicode
  pool: 5

Any help is appreciated.

Edit: here's my docker-compose.yml:

web:
  build: .
  volumes:
    - .:/my_app
  ports:
    - "3000:3000"
  links:
    - postgres
    - redis
    - mailcatcher
  env_file:
    - 'config/application.yml'
postgres:
  image: postgres:9.4
  ports:
    - "5432"
  env_file:
    - 'config/database.yml'
redis:
  image: redis:3.0.6
mailcatcher:
  image: schickling/mailcatcher
  ports:
    - "1080:1080"

Answer 1

Postgres image expects POSTGRES_USER, POSTGRES_PASSWORD and POSTGRES_DB to be provided. Otherwise it will use default values. Create a .env file in the root directory of your project:

# .env file
POSTGRES_USER=testuser
POSTGRES_PASSWORD=testpass
POSTGRES_DB=db_development

and change you docker-compose file as:

  web:
    build: .
    volumes:
      - .:/my_app
    ports:
      - 3000:3000
    depends_on:
      - postgres
      - redis
      - mailcatcher
  postgres:
    image: postgres:9.4
    ports:
      - 5432:5432
    env_file: .env
  redis:
    image: redis:3.0.6
  mailcatcher:
    image: schickling/mailcatcher
    ports:
      - 1080:1080

You could also provide the environment variables without .env file:

  web:
    build: .
    volumes:
      - .:/my_app
    ports:
      - 3000:3000
    depends_on:
      - postgres
      - redis
      - mailcatcher
  postgres:
    image: postgres:9.4
    ports:
      - 5432:5432
    environment:
      - POSTGRES_USER=testuser
      - POSTGRES_PASSWORD=testpass
      - POSTGRES_DB=db_development
  redis:
    image: redis:3.0.6
  mailcatcher:
    image: schickling/mailcatcher
    ports:
      - 1080:1080

and update your database.yml file:

default: &default
  adapter: postgresql
  encoding: unicode
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
  username: <%= ENV['POSTGRES_USER'] %>
  password: <%= ENV['POSTGRES_PASSWORD'] %>
  host: postgres
development:
  <<: *default
  database: db_development
test:
  <<: *default
  database: db_test
production:
  <<: *default
  database: db_production
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 25 } %>

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