如何：DataMapper中的單表繼承？

Question

我學習西納特拉（1.3.2），並選擇使用的DataMapper（1.2.0）作爲ORM和內存的SQLite（1.3.6）DB啓動。
兩個模型Books和Downloads共享大多數屬性，因此我研究了DataMapper中的STI（單表繼承）的聲明模型。閱讀the docs，這似乎是小菜一碟，感謝Types::Discriminator。

我抽象所有常見的爲DownloadableResource：

class DownloadableResource 
    include DataMapper::Resource 

    property :id,   Serial 
    property :created_at, DateTime 
    property :modified_at, DateTime 
    property :active,  Boolean, default: true 

    property :position,  Integer 

    property :title,  String, required: true 
    property :url,   URI,  required: true 
    property :description, Text,  required: true 

    property :type,   Discriminator 
end 

繼the example，我還以爲它只是那麼容易，因爲指定需要擴展的內容：

class Book < DownloadableResource 
    property :cover_url, URI 
    property :authors,  String, required: true, length: 255 
end 

和

class Download < DownloadableResource 
    property :icon_url,  URI 
end 

但這給了我以下錯誤：

DataObjects::SyntaxError: duplicate column name: id (code: 1, sql state: , query: ALTER TABLE "downloadable_resources" ADD COLUMN "id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, uri: sqlite3::memory:?scheme=sqlite&user=&password=&host=&port=&query=&fragment=&adapter=sqlite3&path=:memory:)

同時去除ID，生成另一個（明顯）錯誤：

DataMapper::IncompleteModelError: DownloadableResource must have a key to be valid

我解決此得到了通過添加include DataMapper::Resource既Book和Download，然後Book需要的關鍵是有效的，現在看像這樣：

class Book < DownloadableResource 
    include DataMapper::Resource 

    property :id,   Serial 

    property :cover_url, URI 
    property :authors,  String, required: true, length: 255 
end 

同去的Download，但現在的問題是：

DataObjects::SyntaxError: duplicate column name: id (code: 1, sql state: , query: ALTER TABLE "books" ADD COLUMN "id" INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, uri: sqlite3::memory:?scheme=sqlite&user=&password=&host=&port=&query=&fragment=&adapter=sqlite3&path=:memory:)

開始覺得我在圈子裏，在DataMapper中實現單表繼承的正確方法是什麼？

---

PS：我已經看過

• DataMapper - Single Table Inheritance和
• Ruby Datamapper table inheritance with associations

，但我還是有這個問題。

Answer 1

我會推薦這種方法：

module DownloadableResource 

    def self.included base 
    base.class_eval do 
     include DataMapper::Resource 

     property :created_at, DateTime 
     property :modified_at, DateTime 
     property :active,  base::Boolean, default: true 

     property :position,  Integer 

     property :title,  String, required: true 
     property :url,   base::URI,  required: true 
     property :description, base::Text,  required: true 

     property :type,   base::Discriminator 
    end 
    end 
end 

class Book 
    include DownloadableResource 
    property :id, Serial 
    # other properties 
end 

class Download 
    include DownloadableResource 
    property :id, Serial 
    # other properties 
end