Best way to create custom config options for my Rails app?

Rails 3 version of initialiser code is as follows (RAILS_ROOT & RAILS_ENV are deprecated)

APP_CONFIG = YAML.load_file(Rails.root.join('config', 'config.yml'))[Rails.env]

Also, Ruby 1.9.3 uses Psych which makes merge keys case sensitive so you'll need to change your config file to take that into account, e.g.

defaults: &DEFAULTS
  audiocast_uri_format: http://blablalba/blabbitybla/yadda

development:
  <<: *DEFAULTS

test:
  <<: *DEFAULTS

production:
  <<: *DEFAULTS

For general application configuration that doesn't need to be stored in a database table, I like to create a config.yml file within the config directory. For your example, it might look like this:

defaults: &defaults
  audiocast_uri_format: http://blablalba/blabbitybla/yadda

development:
  <<: *defaults

test:
  <<: *defaults

production:
  <<: *defaults

This configuration file gets loaded from a custom initializer in config/initializers:

# Rails 2
APP_CONFIG = YAML.load_file("#{RAILS_ROOT}/config/config.yml")[RAILS_ENV]

# Rails 3+
APP_CONFIG = YAML.load_file(Rails.root.join('config/config.yml'))[Rails.env]

If you're using Rails 3, ensure you don't accidentally add a leading slash to your relative config path.

You can then retrieve the value using:

uri_format = APP_CONFIG['audiocast_uri_format']

See this Railscast for full details.

Step 1: Create config/initializers/appconfig.rb

require 'ostruct'
require 'yaml'

all_config = YAML.load_file("#{Rails.root}/config/config.yml") || {}
env_config = all_config[Rails.env] || {}
AppConfig = OpenStruct.new(env_config)

Step 2: Create config/config.yml

common: &common
  facebook:
    key: 'asdjhasxas'
    secret : 'xyz'
  twitter:
    key: 'asdjhasxas'
    secret : 'abx'

development:
  <<: *common

test:
  <<: *common

production:
  <<: *common

Step 3: Get constants anywhere in the code

facebook_key = AppConfig.facebook['key']
twitter_key  = AppConfig.twitter['key']

Rails >= 4.2

Just create a YAML file into config/ directory, for example: config/neo4j.yml.

Content of neo4j.yml can be somthing like below(For simplicity, I used default for all environments):

default: &default
  host: localhost
  port: 7474
  username: neo4j
  password: root

development:
  <<: *default

test:
  <<: *default

production:
  <<: *default

in config/application.rb:

module MyApp
  class Application < Rails::Application
    config.neo4j = config_for(:neo4j)
  end
end

Now, your custom config is accessible like below:

Rails.configuration.neo4j['host'] #=>localhost
Rails.configuration.neo4j['port'] #=>7474

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More info

Rails official API document describes config_for method as:

Convenience for loading config/foo.yml for the current Rails env.

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If you do not want to use a yaml file

As Rails official guide says:

You can configure your own code through the Rails configuration object with custom configuration under the config.x property.

Example

config.x.payment_processing.schedule = :daily
config.x.payment_processing.retries  = 3
config.x.super_debugger = true

These configuration points are then available through the configuration object:

Rails.configuration.x.payment_processing.schedule # => :daily
Rails.configuration.x.payment_processing.retries  # => 3
Rails.configuration.x.super_debugger              # => true
Rails.configuration.x.super_debugger.not_set      # => nil

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Official Reference for config_for method | Official Rails Guide