# config/database.yml
default: &default
  adapter: mysql2
  encoding: utf8
  pool: 5
  username: <%= ENV['DATABASE_USERNAME'] %>
  password: <%= ENV['DATABASE_PASSWORD'] %>
  socket: /var/run/mysqld/mysqld.sock

development:
  <<: *default
database: <%= ENV['DATABASE_NAME'] %>
test:
  <<: *default
database: <%= ENV['DATABASE_NAME'] %>
production:
  <<: *default
  database: <%= ENV['DATABASE_NAME'] %>
  username: <%= ENV['DATABASE_USERNAME'] %>
password: <%= ENV['DATABASE_PASSWORD'] %>

# config/application.rb
config = YAML.load(File.read(File.expand_path('../application.yml', __FILE__)))
config.merge! config.fetch(Rails.env, {})
config.each do |key, value|
    ENV[key] = value.to_s unless value.kind_of? Hash
end

# config/application.yml
development:

  DATABASE_USERNAME: "username goes here"
  DATABASE_PASSWORD: "password goes here"
  DATABASE_NAME: "database name goes here"

  SECRET_KEY_BASE: "secret key goes here"

test:

  DATABASE_USERNAME: "username goes here"
  DATABASE_PASSWORD: "password goes here"
  DATABASE_NAME: "database name goes here"

  SECRET_KEY_BASE: "secret key goes here"

production:

  DATABASE_USERNAME: "username goes here"
  DATABASE_PASSWORD: "password goes here"
  DATABASE_NAME: "database name goes here"

  SECRET_KEY_BASE: "secret key goes here"

Rails.application.secrets

SECRETS[:my_secret_key]

rails g config:install

config/settings.yml
config/settings/development.yml
config/settings/production.yml
config/settings/test.yml