My app config setup

aaa_load_app_config.yml

# in config/initializers/
# prefixed with 'aaa_' so it's loaded first and app configs can be used in the following initializers

require 'ostruct'

config = File.read(Rails.root.join('config', 'app_config.yml'))

unless config.empty?
  ::AppConfig = OpenStruct.new(YAML.load(ERB.new(config).result).with_indifferent_access)

  %w(development test production).each do |env|
    if (env_config = AppConfig.delete_field(env)).present? && Rails.env == env
      env_config.each do |k, v|
        if AppConfig.send(k).present?
          AppConfig.send(k).merge!(v)
        else
          AppConfig.send([k, '='].join, v)
        end
      end
    end
  end
end

app_config.yml

# in config/
# ERb can be used

s3:
  key: devkey
  secret: devsecret

foo:
  bar:
    baz: <%= 1.day %>

production:

  s3:
    key: realkey
    secret: realkey

wherever.rb

# And this is how you use it in your code

AppConfig.foo[:bar][:baz] # 86400 (24*60*60)
AppConfig.s3[:key] # 'devkey' in development and test environment, 'realkey' in production environment