Seperation of concerns with clean namespaces.
Normal includes and extensions pollute the namespace and share private/protected.
With concern, everything stays in the Concern, can access the concerned and keep namespaced+private.
gem install concern
# a.rb require 'concern' class A concern 'a/acl' def admin? ... end # a/acl.rb # can access concerned methods like `admin?` class A::ACL < Concern def can_access?(vault) admin? and secret end private def secrect ... end A.new.acl.can_access?(BankAccount)
Let no outsider know, and use delegate:
class A class Messages def write ... def read ... end concern 'a/acl', :delegate => true # delegate all public methods concern 'a/messages', :delegate => [:write] end A.new.can_access?(BankAccount) # no more `.acl` needed A.new.write A.new.messages.read
Adding to the concerned, e.g. validations/associations/scopes etc.
class User < ActiveRecord::Base concern 'user/validator', :delegate => true end # user/validator.rb class User::Validator < Concern def self.included(concerned) concerned.class_eval do validates_presence_of :name, :if => :name_seems_legit? end end def name_seems_legit? .. end
More examples can be found in spec/examples