sortable

module Sortable

  def self.included(base)
    base.extend(ClassMethods)
    base.send(:include, InstanceMethods)
    base.send(:class_inheritable_accessor, :sort_attributes)
    base.sort_attributes = []
  end

  module ClassMethods

    def sort_by(*attributes)
      self.sort_attributes = attributes
    end

  end

  module InstanceMethods

    def <=>(other)
      return 1 unless other
      self.class.sort_attributes.each do |attribute|
        a, b = send(attribute) || 0, other.send(attribute) || 0
        return -1 unless a.respond_to?(:'<=>')
        result = a <=> b
        return result if result && !result.zero?
      end
      0
    end

  end

end