database.yml

development: &default
  adapter: mysql2
  username: <%= ENV.fetch('MYSQL_USER',     'foo') %>
  password: <%= ENV.fetch('MYSQL_PASSWORD', 'bar') %>
  host:     <%= ENV.fetch('MYSQL_HOST',     '127.0.0.1') %>
  database: app_development
  encoding: utf8
.....

mongoid.yml

development:
  sessions:
    default:
      database: app_development
      hosts:
        - <%= ENV.fetch('MONGO_HOST', 'localhost') %>:ENV.fetch('MONGO_PORT', 27017) %>
...

sphinx.yml

development: &default
  address: <%= ENV.fetch('SPHINX_HOST', 'localhost') %>
 ...