def make_key class_name
      name = class_name.to_s.pluralize.gsub(/::/, '__').underscore
      only_last_part_plural(name).to_sym
    end
def make_key class_name
      name = class_name.to_s.pluralize.gsub(/::/, '__').underscore
      only_last_part_plural(name).to_sym
    end
creates a key from a class name
 fx UsersRoles becomes :user_roles, where only the last part is pluralised!
 @param [String] the class name