Rails 多态关联与同一模型上的多个关联 [英] Rails Polymorphic Association with multiple associations on the same model

问题描述

我的问题与这个问题基本相同:多态关联与同一模型上的多个关联

My question is essentially the same as this one: Polymorphic Association with multiple associations on the same model

但是，建议/接受的解决方案不起作用，稍后评论者会说明.

However, the proposed/accepted solution does not work, as illustrated by a commenter later.

我有一个在我的应用程序中使用的 Photo 类.一个帖子可以有一张照片.但是，我想重新使用多态关系来添加第二张照片.

I have a Photo class that is used all over my app. A post can have a single photo. However, I want to re-use the polymorphic relationship to add a secondary photo.

之前:

class Photo 
   belongs_to :attachable, :polymorphic => true
end

class Post
   has_one :photo, :as => :attachable, :dependent => :destroy
end

期望:

class Photo 
   belongs_to :attachable, :polymorphic => true
end

class Post
   has_one :photo,           :as => :attachable, :dependent => :destroy
   has_one :secondary_photo, :as => :attachable, :dependent => :destroy
end

但是，这失败了，因为它找不到类SecondaryPhoto".根据我从其他线程中可以看出的情况，我想做:

However, this fails as it cannot find the class "SecondaryPhoto". Based on what I could tell from that other thread, I'd want to do:

   has_one :secondary_photo, :as => :attachable, :class_name => "Photo", :dependent => :destroy

除了调用 Post#secondary_photo 只会返回通过照片关联附加的同一张照片，例如发布#photo === 发布#secondary_photo.查看 SQL，它确实 WHERE type = "Photo" 而不是我想要的SecondaryPhoto"...

Except calling Post#secondary_photo simply returns the same photo that is attached via the Photo association, e.g. Post#photo === Post#secondary_photo. Looking at the SQL, it does WHERE type = "Photo" instead of, say, "SecondaryPhoto" as I'd like...

想法?谢谢！

推荐答案

我已经在我的项目中做到了.

I have done that in my project.

诀窍是照片需要一个列，该列将在 has_one 条件下用于区分主要照片和次要照片.注意这里 :conditions 发生了什么.

The trick is that photos need a column that will be used in has_one condition to distinguish between primary and secondary photos. Pay attention to what happens in :conditions here.

has_one :photo, :as => 'attachable', 
        :conditions => {:photo_type => 'primary_photo'}, :dependent => :destroy

has_one :secondary_photo, :class_name => 'Photo', :as => 'attachable',
        :conditions => {:photo_type => 'secondary_photo'}, :dependent => :destroy

这种方法的美妙之处在于，当您使用 @post.build_photo 创建照片时，photo_type 将自动预先填充相应的类型，例如primary_photo".ActiveRecord 足够聪明，可以做到这一点.

The beauty of this approach is that when you create photos using @post.build_photo, the photo_type will automatically be pre-populated with corresponding type, like 'primary_photo'. ActiveRecord is smart enough to do that.

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