Question

Could someone please explain this code, this is the same blog app explained
on github but I could not understand the use of this part specially the use of name spaces role mask.

There are three roles in this app admin, moderator and author.
Based on the CRUD functionality they are able to edit comments or delete a comment.

         class User < ActiveRecord::Base
                  acts_as_authentic
                  has_many :articles
                  has_many :comments

                  named_scope :with_role, lambda { |role| {:conditions => "roles_mask & #{2**ROLES.index(role.to_s)} > 0"} }

                  ROLES = %w[admin moderator author]

                  def roles=(roles)
                    self.roles_mask = (roles & ROLES).map { |r| 2**ROLES.index(r) }.sum
                  end

                  def roles
                    ROLES.reject { |r| ((roles_mask || 0) & 2**ROLES.index(r)).zero? }
                  end

                  def role_symbols
                    roles.map(&:to_sym)
                  end
                end

Answer 1

The role_mask is a bitfield. Whenever roles get assigned to the User, the role_mask automatically gets updated. The possible values for the roles mask and respective roles:

111: author, moderator, admin
110: author, moderator
101: author, admin
100: author
011: moderator, admin
010: moderator
001: admin

The bitfield is used to quickly search the database for users with a specific role. This is done by applying & operator on the role_mask with the bitfield of the role you are looking for. For example, to get all users who are authors, the with_role scope does a database query for all records for which roles_mask & 100 is true.