Question

我正在学习 Sinatra （1.3.2）并选择使用 DataMapper （1.2.0）作为ORM和内存 SQLite （1.3.6）DB开始。
两个模型Books和Downloads共享大多数属性，因此我研究了在DataMapper中声明STI（单表继承）的模型。阅读the docs，感谢Types::Discriminator，这似乎是小菜一碟。

我将所有常见问题抽象为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

关注the example，我认为这就像指定需要扩展的内容一样简单：

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

和

class Download < DownloadableResource
  property :icon_url,     URI
end

但这给了我以下错误：

DataObjects :: SyntaxError：重复列名：id （代码：1，sql state :, query：ALTER TABLE“ downloadable_resources ”ADD COLUMN“id”INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT，uri：sqlite3 :: memory：？scheme = sqlite＆amp; user =＆amp; password =＆amp; host =＆amp; port =＆amp; query =＆amp; fragment =＆amp; adapter = sqlite3＆amp; path =：memory：）

删除id时产生另一个（明显的）错误：

DataMapper :: IncompleteModelError：DownloadableResource必须有一个有效的密钥

我通过向include DataMapper::Resource和Book添加Download来解决这个问题，然后Book需要一个有效的密钥，现在看起来像这样：

class Book < DownloadableResource
  include DataMapper::Resource

  property :id,           Serial

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

同样适用于Download，但现在的问题是：

DataObjects :: SyntaxError：重复列名：id （代码：1，sql state :, query：ALTER TABLE“ books ”ADD COLUMN“id”INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT，uri：sqlite3 :: memory：？scheme = sqlite＆amp; user =＆amp; password =＆amp; host =＆amp; port =＆amp; query =＆amp; fragment =＆amp; adapter = sqlite3＆amp; path =：memory：）

开始觉得我要进入圈子，在DataMapper中实现单表继承的正确方法是什么？

PS：我看过了

但我仍有这个问题。

Answer 1

我会推荐这种方法：

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

如何：DataMapper中的单表继承？

1 个答案: