config.rb

> Rails.application.config_for(:database)
 => {"adapter"=>"mysql2", "encoding"=>"utf8", "reconnect"=>false, "database"=>"listia_development", "pool"=>5, "username"=>"listia", "socket"=>"/tmp/mysql.sock"}

> h = Hashie::Mash.new(Rails.application.config_for(:database))
 => #<Hashie::Mash adapter="mysql2" database="listia_development" encoding="utf8" pool=5 reconnect=false socket="/tmp/mysql.sock" username="listia">
> h.adapter
 => "mysql2"
> h.database
 => "listia_development"

   class Application < Rails::Application
    config.x.foo = Hashie::Mash.new(Rails.application.config_for(:bar))
    ...
    
   # Then you could use Rails.configuration.x.foo.yyy to access the config value of current environment