rails-4-dot-2-config-for.org

Rails 4.2 config_for

WARNING: This is old and likely obsolete.

• published date: 2015-01-20 07:30
• keywords: [“configuration”, “rails”, “swaac”]

There have been a lot of blog posts and articles written on how to add application-specific configurations to a Rails app. In version 4.2, there is a method called config_for which makes a lot of that obsolete, simplifying the results.

A common Rails idiom

A seemingly very common idiom is to arrange appliation configuration values in YAML+ERB files under the config directory in a Rails app. Then, in an initializer, read in the file, process it through ERB, then YAML extract the set appropriate to the running environment, and save it in a constant or global:

config/app_config.yml

default: &default
  service_url: https://my_service.example.com

development:
  <<: *default

test:
  <<: *default

production:
  service_url: <%= ENV['APP_SERVICE_URL'] %>

config/initializers/app_config.rb

require 'yaml'
require 'erb'

APP_CONFIG = YAML.load(ERB.new(File.read(Rails.root.join('config','app_config.yml'))).result)[Rails.env] || {}

Enter config_for at Rails 4.2

=config_for= is a method on Rails.application. It can be called bar within all the normal configuration contexts, such as inside config/environments/development.rb and such.

What it does is fairly straightforward, reading a YAML+ERB file from the config directory, returning the appropriate section based on the current Rails environment.

It’s a simple enough implementation: https://github.com/rails/rails/blob/1d1239d32856b32b19c04edd17d0dd0d47611586/railties/lib/rails/application.rb#L226 (Note of course this is a volatile link). It’s implemented in Rails::Application#config_for.

You pass in a symbol that corresponds to a file name under config, so config_for(:app_config) loads up the appropriate environment variables from config/app_config.yml.

Now, your initializer can become simply:

config/initializers/app_config.rb

APP_CONFIG = Rails.application.config_for(:app_config)

Example

I’ve an example github repo at https://github.com/tamouse/test_config_for. Feel free to fork it and play with it yourself.