Bit Mask Helper for ActiveRecord

qualification_mask.rb

module QualificationMask
  def bitmask_field_for(field, options)
    array = options.delete(:with).to_s
    field = field.to_s
    methods = field.gsub /_mask/, ''
    
    class_eval %(
      def #{methods}=(#{methods})
        #{methods} = [#{methods}] if not #{methods}.is_a?(Array)
        self.#{field} = (#{methods} & #{array}).map { |r| 2**#{array}.index(r) }.sum
      end

      def #{methods}
        list = #{array}.reject { |r| ((self.#{field} || 0) & 2**#{array}.index(r)).zero? }
      end
      
      scope :with_#{methods}, lambda { |r|
        r = [r] if not r.is_a?(Array)
        conditions = []
        r.each do |f|
          conditions.push("\#{field} & \#{2**#{array}.index(f)} > 0")
        end
        {
          :conditions => conditions.join(' and ')
        }
      }
    )
  end
end

user.rb

class User < ActiveRecord::Base
  extend QualificationMask
  QUALIFICATIONS = ['ALS', 'Medic', 'First Responder']
  bitmask_field_for :qualifications_mask, :with => 'QUALIFICATIONS'
end

user = User.new
user.qualifications = User::QUALIFICATIONS
user.qualifications # => ['ALS', 'Medic', 'First Responder']
user.with_qualifications('ALS').count # => 1
user.with_qualifications(['ALS', 'Medic']).count # => 1