module Categorizable

  module ClassMethods

    def acts_as_categorizable
      has_many :audience_scopes, :as => :audienciable
      has_many :audience_categories, :through => :audience_scopes

      has_many :areas, :through => :audience_scopes

      send :include, InstanceMethods
    end

    def for_user(user)
      ...
    end

  end

  module InstanceMethods

    def related
      ...
    end

  end
end

ActiveRecord::Base.send :include, Categorizable