def only_last_part_plural cls_name 
      parts = cls_name.split('_')
      name = parts.inject([]) do |res, part|
        res << (part != parts.last ? part.singularize : part)
        res
      end.join('_')          
    end
def only_last_part_plural cls_name 
      parts = cls_name.split('_')
      name = parts.inject([]) do |res, part|
        res << (part != parts.last ? part.singularize : part)
        res
      end.join('_')          
    end
Takes a composite name and makes a nice sounding key that follows the Rails conventions
 fx UsersRoles becomes :user_roles, where only the last part is pluralised!
 @param [String] the class name, fx UsersRoles