ActiveRecord通过针对STI类的作用域构建错误类的实例 [英] ActiveRecord builds instance of wrong class through a scope targeting an STI class

问题描述

我希望能够在通过STI类型定位到特定模型类的作用域上调用build方法，并让ActiveRecord构建正确类的实例.

I would like to be able to call the build method on a scope that targets a certain class of model via its STI type, and have ActiveRecord build an instance of the correct class.

class LineItem < ActiveRecord::Base
  scope :discount, where(type: 'DiscountLineItem')
end

class DiscountLineItem < LineItem; end

> LineItem.discount.build # Expect an instance of DiscountLineItem here
=> #<LineItem ...>

在这里，我期望的是DiscountLineItem的实例，而不是LineItem的实例.

Here, I expected an instance of DiscountLineItem, not an instance of LineItem.

推荐答案

即使ActiveRecord不会将对象实例化为正确的类，它也会正确设置类型.基本上，您有两种解决方法:

Even though ActiveRecord doesn't instantiate the object as the right class, it does set the type correctly. You basically have two ways around this:

1)创建对象，然后从数据库中重新加载它:

1) Create the object and then reload it from the database:

item = LineItem.discount.create(attrs...)
item = LineItem.find(item.id)

2)使用STI类并直接从其构建对象:

2) Use the STI class and build the object directly from it:

DiscountLineItem.build

ActiveRecord可以做的所有事情，这似乎是一种毫无意义的限制，并且可能很难改变.现在，您激起了我的兴趣:)

With all that ActiveRecord can do, this does seem like kind of a senseless limitation and might not be too hard to change. Now you've piqued my interested :)

更新:

这最近被添加到Rails 4.0 中，并带有以下提交消息:

This was recently added to Rails 4.0 with the following commit message:

允许您执行BaseClass.new(:type =>"SubClass")以及 parent.children.build(:type =>"SubClass")或parent.build_child 初始化STI子类.确保类名是有效的 阶级是在超阶级的祖先中 协会在期待.

Allows you to do BaseClass.new(:type => "SubClass") as well as parent.children.build(:type => "SubClass") or parent.build_child to initialize an STI subclass. Ensures that the class name is a valid class and that it is in the ancestors of the super class that the association is expecting.

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