What is mattr_accessor in a Rails module?

Question

I couldn\'t really find this in Rails documentation but it seems like \'mattr_accessor\' is the Module corollary for \'attr_accesso

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 = "admin@example.com"
>> Config.hostname # => "example.com"
>> Config.admin_email # => "admin@example.com"