Can't connect to postgres in docker

Question

I am trying to connect Postgres DB to a Rails app using docker-compose as following

docker-compose.yml

version: '3.8'
services:
  db:
    image: postgres:12.1
    volumes:
      - pg-data:/var/lib/postgresql/data
    environment:
      POSTGRES_PASSWORD: "password"
  web:
    build: .
    command: bash -c "rm -f tmp/pids/server.pid && bundle exec rails s -p 3000 -b '0.0.0.0'"
    volumes:
      - .:/rails6
    ports:
      - "3000:3000"
    depends_on:
      - db
volumes:
  pg-data:

Dockerfile

FROM ruby:2.5

RUN curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add -
RUN echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list

RUN apt-get update -qq && apt-get install -y nodejs postgresql-client yarn

RUN mkdir /rails6
WORKDIR /rails6

RUN gem install bundler:2

COPY package.json yarn.lock ./
RUN yarn install --check-files

COPY Gemfile* ./
RUN bundle install

COPY . /rails6

EXPOSE 3000

# Start the main process.
CMD ["rails", "server", "-b", "0.0.0.0"]

But when I run docker-compose run web rake db:create I get following error.

ActiveRecord::NoDatabaseError: could not connect to server: No such file or directory
    Is the server running locally and accepting
    connections on Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/connection_adapters/postgresql_adapter.rb:50:in `rescue in postgresql_connection'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/connection_adapters/postgresql_adapter.rb:33:in `postgresql_connection'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/connection_adapters/abstract/connection_pool.rb:887:in `new_connection'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/connection_adapters/abstract/connection_pool.rb:931:in `checkout_new_connection'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/connection_adapters/abstract/connection_pool.rb:910:in `try_to_checkout_new_connection'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/connection_adapters/abstract/connection_pool.rb:871:in `acquire_connection'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/connection_adapters/abstract/connection_pool.rb:593:in `checkout'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/connection_adapters/abstract/connection_pool.rb:437:in `connection'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/connection_adapters/abstract/connection_pool.rb:1119:in `retrieve_connection'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/connection_handling.rb:221:in `retrieve_connection'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/connection_handling.rb:189:in `connection'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/tasks/postgresql_database_tasks.rb:12:in `connection'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/tasks/postgresql_database_tasks.rb:21:in `create'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/tasks/database_tasks.rb:126:in `create'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/tasks/database_tasks.rb:185:in `block in create_current'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/tasks/database_tasks.rb:479:in `block (2 levels) in each_current_configuration'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/tasks/database_tasks.rb:476:in `each'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/tasks/database_tasks.rb:476:in `block in each_current_configuration'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/tasks/database_tasks.rb:475:in `each'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/tasks/database_tasks.rb:475:in `each_current_configuration'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/tasks/database_tasks.rb:184:in `create_current'
/usr/local/bundle/gems/activerecord-6.0.3/lib/active_record/railties/databases.rake:39:in `block (2 levels) in <top (required)>'

Am I missing something?

Answer 1

dI had a very similar issue and then realized that I had not added my db host to the config/database.yml

default: &default
    adapter: postgresql
    encoding: unicode
    host: db
    # For details on connection pooling, see Rails configuration guide
    # https://guides.rubyonrails.org/configuring.html#database-pooling
    pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
    username: <%= ENV['MYAPP_DATABASE_USERNAME'] %>
    password: <%= ENV['MYAPP_DATABASE_PASSWORD'] %>

The value for "host" is db because it is the service name in the docker-compose.yml

...
services:
  db: # <--- right here
    image: postgres:12.1
    volumes:
      - pg-data:/var/lib/postgresql/data
    environment:
      POSTGRES_PASSWORD: "password"
...