RailsでSingle Table Inheritance

_description.markdown

単一テーブル継承の実装

参考

• http://www.martinfowler.com/eaaCatalog/singleTableInheritance.html
• http://api.rubyonrails.org/classes/ActiveRecord/Base.html#class-ActiveRecord::Base-label-Single+table+inheritance

モデル

• Message
• GoodMessage(Messageを継承)
• BadMessage(Messageを継承)

データの保存

Message.create(title: 1)
GoodMessage.create(title: 'good', good: 111)
BadMessage.create(title: 'bad', bad: 222)

mysql> select * from messages;
+----+-------+-------------+------+------+
| id | title | type        | good | bad  |
+----+-------+-------------+------+------+
|  1 | 1     | NULL        | NULL | NULL |
|  2 | good  | GoodMessage | 111  | NULL |
|  3 | bad   | BadMessage  | NULL | 222  |
+----+-------+-------------+------+------+

子クラスでデータを保存すると、typeにクラス名が自動で入る。

bad_message.rb

class BadMessage < Message
end                       

create_messages.rb

class CreateMessages < ActiveRecord::Migration
  def change
    create_table :messages do |t|
      t.string :title, null: false
      t.string :type                          
      t.string :good
      t.string :bad
    end
  end
end

good_message.rb

class GoodMessage < Message
end

message.rb

class Message < ActiveRecord::Base
end