Can't run rake db:create in Dockerfile with docker-compose

Question

I have a Dockerfile and docker-compose.yml like in the tutorial except I'm starting with an existing app.

My docker-compose.yml looks like:

db:   image: postgres   ports:     - "5432" web:   build: .   command: bundle exec rails s -p 3000 -b '0.0.0.0'   volumes:     - .:/myapp   ports:     - "3030:3030"   links:     - db 

and Dockerfile:

FROM ruby:2.1.4 RUN apt-get update -qq && apt-get install -y build-essential libpq-dev nodejs RUN mkdir /myapp WORKDIR /myapp ADD Gemfile /myapp/Gemfile RUN bundle install ADD . /myapp # RUN bundle exec rake db:create # RUN bundle exec rake db:migrate # RUN bundle exec rake db:seed 

and database.yml

development:   adapter: postgresql   encoding: utf8   database: myapp_development   host: db   pool: 5   username: postgres   password: 

As you can see, I commented RUN bundle exec rake db:create because I was receiving an error:

could not translate host name "db" to address: Name or service not known /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/connection_adapters/postgresql_adapter.rb:1222:in `initialize' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/connection_adapters/postgresql_adapter.rb:1222:in `new' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/connection_adapters/postgresql_adapter.rb:1222:in `connect' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/connection_adapters/postgresql_adapter.rb:324:in `initialize' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/connection_adapters/postgresql_adapter.rb:28:in `new' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/connection_adapters/postgresql_adapter.rb:28:in `postgresql_connection' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/connection_adapters/abstract/connection_pool.rb:315:in `new_connection' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/connection_adapters/abstract/connection_pool.rb:325:in `checkout_new_connection' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/connection_adapters/abstract/connection_pool.rb:247:in `block (2 levels) in checkout' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/connection_adapters/abstract/connection_pool.rb:242:in `loop' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/connection_adapters/abstract/connection_pool.rb:242:in `block in checkout' /usr/local/lib/ruby/2.1.0/monitor.rb:211:in `mon_synchronize' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/connection_adapters/abstract/connection_pool.rb:239:in `checkout' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/connection_adapters/abstract/connection_pool.rb:102:in `block in connection' /usr/local/lib/ruby/2.1.0/monitor.rb:211:in `mon_synchronize' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/connection_adapters/abstract/connection_pool.rb:101:in `connection' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/connection_adapters/abstract/connection_pool.rb:410:in `retrieve_connection' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/connection_adapters/abstract/connection_specification.rb:171:in `retrieve_connection' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/connection_adapters/abstract/connection_specification.rb:145:in `connection' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/railties/databases.rake:144:in `rescue in create_database' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/railties/databases.rake:85:in `create_database' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/railties/databases.rake:62:in `block (3 levels) in <top (required)>' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/railties/databases.rake:62:in `each' /usr/local/bundle/gems/activerecord-3.2.21/lib/active_record/railties/databases.rake:62:in `block (2 levels) in <top (required)>' /usr/local/bundle/gems/rake-10.4.2/lib/rake/task.rb:240:in `call' /usr/local/bundle/gems/rake-10.4.2/lib/rake/task.rb:240:in `block in execute' /usr/local/bundle/gems/rake-10.4.2/lib/rake/task.rb:235:in `each' /usr/local/bundle/gems/rake-10.4.2/lib/rake/task.rb:235:in `execute' /usr/local/bundle/gems/rake-10.4.2/lib/rake/task.rb:179:in `block in invoke_with_call_chain' /usr/local/lib/ruby/2.1.0/monitor.rb:211:in `mon_synchronize' /usr/local/bundle/gems/rake-10.4.2/lib/rake/task.rb:172:in `invoke_with_call_chain' /usr/local/bundle/gems/rake-10.4.2/lib/rake/task.rb:165:in `invoke' /usr/local/bundle/gems/rake-10.4.2/lib/rake/application.rb:150:in `invoke_task' /usr/local/bundle/gems/rake-10.4.2/lib/rake/application.rb:106:in `block (2 levels) in top_level' /usr/local/bundle/gems/rake-10.4.2/lib/rake/application.rb:106:in `each' /usr/local/bundle/gems/rake-10.4.2/lib/rake/application.rb:106:in `block in top_level' /usr/local/bundle/gems/rake-10.4.2/lib/rake/application.rb:115:in `run_with_threads' /usr/local/bundle/gems/rake-10.4.2/lib/rake/application.rb:100:in `top_level' /usr/local/bundle/gems/rake-10.4.2/lib/rake/application.rb:78:in `block in run' /usr/local/bundle/gems/rake-10.4.2/lib/rake/application.rb:176:in `standard_exception_handling' /usr/local/bundle/gems/rake-10.4.2/lib/rake/application.rb:75:in `run' /usr/local/bundle/gems/rake-10.4.2/bin/rake:33:in `<top (required)>' /usr/local/bundle/bin/rake:16:in `load' /usr/local/bundle/bin/rake:16:in `<main>' Couldn't create database for {"adapter"=>"postgresql", "encoding"=>"utf8", "database"=>"myapp_development", "host"=>"db", "pool"=>5, "username"=>"postgres", "password"=>nil} 

Instead I had to docker-compose build then docker-compose run web rake db:create etc.

Why can't I have the db-creation/migration in the Dockerfile. It would be so much clearner. Can I achieve that?

Answer 1

When your web image is built (following the instructions of the Dockerfile), it doesn't have a connection to a db container.

The webserver and database images are independents and the containers are linked when you launch them (following the definitions of the docker-compose.yml file).

You cannot link to a container during the image build because it would break the principle that an image build must be reproducible. Similarly, you cannot mount a volume from the host machine during an image build neither.

The docker-compose run web rake db:create command you used is a correct way to initalize the database.

Alternatively, you could launch the containers normaly with docker-compose, and then use the docker exec command to execute rake db:create in the web container.