Question

I couldn’t really find this in Rails documentation but it seems like ‘mattr_accessor’ is the Module corollary for ‘attr_accessor’ (getter & setter) in a normal Ruby class.

Eg. in a class

class User   attr_accessor :name    def set_fullname     @name = '#{self.first_name} #{self.last_name}'   end end 

Eg. in a module

module Authentication   mattr_accessor :current_user    def login     @current_user = session[:user_id] || nil   end end 

This helper method is provided by ActiveSupport.

Answer 1

Rails extends Ruby with both mattr_accessor (Module accessor) and cattr_accessor (as well as _reader/_writer versions). As Ruby’s attr_accessor generates getter/setter methods for instances, cattr/mattr_accessor provide getter/setter methods at the class or module level. Thus:

module Config   mattr_accessor :hostname   mattr_accessor :admin_email end 

is short for:

module Config   def self.hostname     @hostname   end   def self.hostname=(hostname)     @hostname = hostname   end   def self.admin_email     @admin_email   end   def self.admin_email=(admin_email)     @admin_email = admin_email   end end 

Both versions allow you to access the module-level variables like so:

>> Config.hostname = 'example.com' >> Config.admin_email = '[e-mail]' >> Config.hostname # => 'example.com' >> Config.admin_email # => '[e-mail]'