问题描述
如何实现对活动记录的继承?
How to implement inheritance with active records?
例如，我想要一个类 Animal、类 Dog 和类 Cat.
For example, I want a class Animal, class Dog, and class Cat.
模型和数据库表的映射是怎样的?
How would the model and the database table mapping be?
推荐答案
Rails 支持单表继承.
Rails supports Single Table Inheritance.
来自 AR 文档:
Active Record 允许继承将类的名称存储在一个默认情况下命名为类型"的列(可以通过覆盖改变Base.inheritance_column).这意味着看起来像这样的继承:
class Company < ActiveRecord::Base; end
class Firm < Company; end
class Client < Company; end
class PriorityClient < Client; end
当你执行 Firm.create(:name =>37signals")，这个记录将是保存在公司表中的类型=公司".然后您可以使用 Company.find(:first, "name= ‘37signals’") 它将返回一个 Firm 对象.
When you do Firm.create(:name => "37signals"), this record will be saved in the companies table with type = "Firm". You can then fetch this row again using Company.find(:first, "name = ‘37signals’") and it will return a Firm object.
如果您没有类型列在您的表中定义，单表不会触发继承.在这种情况下，它会正常工作没有特殊魔法的子类区分它们或使用 find 重新加载正确的类型.
If you don‘t have a type column defined in your table, single-table inheritance won‘t be triggered. In that case, it‘ll work just like normal subclasses with no special magic for differentiating between them or reloading the right type with find.
这里有一个很好的教程:http://juixe.com/techknow/index.php/2006/06/03/rails-single-table-inheritance/
A pretty good tutorial is here: http://juixe.com/techknow/index.php/2006/06/03/rails-single-table-inheritance/
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