Ruby on Rails
Inheritance

ActiveRecord provides a single-table inheritance model. Child classes are automatically persisted in the parents table using the “type” column (just a varchar) to figure out which class the data belongs to.

Example using Inheritance

Database schema (MySQL):

CREATE TABLE `topics` (
  `id` int(11) NOT NULL auto_increment,
  `title` varchar(255) default NULL,
  `author_name` varchar(255) default NULL,
  `author_email_address` varchar(255) default NULL,
  `written_on` datetime default NULL,
  `last_read` date default NULL,
  `content` text,
  `approved` tinyint(1) default 1,
  `reply_count` int(11) default NULL,
  `parent_id` int(11) default NULL,
  `type` varchar(50) default NULL,
  PRIMARY KEY  (`id`)
);

Model code:

require 'active_record'
class Topic < ActiveRecord::Base
  def parent
    self.class.find(parent_id)
  end
  protected
    def before_destroy
      self.class.delete_all "parent_id = #{id}"
    end
end
class Reply < Topic
end

Associations in the above model: