使用delegate with has_many在Rails？ [英] Using Delegate With has_many In Rails?

问题描述

我们有2个模特&连接模型：

 ＃app / models / message.rb 
类消息< ActiveRecord :: Base 
 has_many：image_messages 
 has_many：images，through：：image_messages 
 end 
 
＃app / models / image.rb 
 Class Image < ActiveRecord :: Base 
 has_many：image_messages 
 has_many：messages，through：：image_messages 
 end 
 
＃app / models / image_message.rb 
 Class ImageMes​​sage < ActiveRecord :: Base 
 belongs_to：image 
 belongs_to：message 
 end 
  

< hr>

额外属性

我们希望从连接模型中提取额外的属性（ ImageMes​​sage ），并在 Message 模型中访问它们：

  @ message.image_messages.first.caption＃ - >现在发生什么
 @ message.images.first.caption＃ - >我们需要
  

我们已经使用 select 方法时声明的关联：

 ＃app / models / message.rb 
 has_many：images， > {select（＃{Image.table_name}。*，＃{ImageMes​​sage.table_name} .caption AS caption）}，class_name：'Image'，through：：image_messages，dependent：：destroy 
  
 
 
 
 
 
 
 代表 
 
 
 
我们刚刚发现 委托 方法，这正是这需要的。但是，它只适用于 has_one 和 belongs_to 关联
 
 
 
我们只是使用一个单一的协会，但似乎它不工作的集合（只需要你到一个公共方法）
 
 
 
 
 
 
 问题 
 
 
 
你知道我们可以返回 .caption  Image 模型从 ImageMes​​sage 连接模型中创建c $ c>属性？
 
 
 
我们目前有：
 ＃app / models / image.rb 
 ; ActiveRecord :: Base 
 has_many：image_messages 
 has_many：messages，through：：image_messages 
 
 delegate：caption，to：：image_messages，allow_nil：true 
 end 
 
＃app / models / image_message.rb 
 Class ImageMes​​sage< ActiveRecord :: Base 
 belongs_to：image 
 belongs_to：message 
 
 def self.caption＃ - >只有类方法
＃我们在这里放吗？ 
 end 
 end 
  
 
 
 
 
 
 
 更新 
 
 
 
感谢Billy Chan（对于实例方法的想法），我们已经非常 
 ＃app / models / image.rb 
 Class Image< ActiveRecord :: Base 
 #Caption 
 def caption 
 self.image_messages.to_a 
 end 
 end 
 
＃app / views / messages / show.html.erb 
<％= @ message.images.each_with_index do | i，index | ％> 
<％= i.caption [index] [：caption]％> ＃ - >工作，但超粗略
<％end％> 
  
任何重构的方式，特别是让每次 .caption 被调用，它返回该记录的 image_message.caption 值？ 
解决方案
 委托只是等效实例方法的简写。这不是所有的解决方案，甚至有一些争论，它不是那么明确。 
 
 
 
当简单委托不能使用时，可以使用实例方法。
 
 
  
 def image_message_caption 
 image_messages.class caption 
 end 
  
 
 
任何关联都是不必要的。 ImageMes​​sage的类方法caption更像一个常量，可以直接引用它。
  def image_message_caption 
 ImageMes​​sage.caption 
 end 
  
 
We've got 2 models & a join model:
#app/models/message.rb
Class Message < ActiveRecord::Base
    has_many :image_messages
    has_many :images, through: :image_messages
end

#app/models/image.rb
Class Image < ActiveRecord::Base
    has_many :image_messages
    has_many :messages, through: :image_messages
end

#app/models/image_message.rb
Class ImageMessage < ActiveRecord::Base
    belongs_to :image
    belongs_to :message
end




Extra Attributes

We're looking to extract the extra attributes from the join model (ImageMessage) and have them accessible in the Message model:
@message.image_messages.first.caption # -> what happens now    
@message.images.first.caption #-> we want
We've already achieved this using the select method when declaring the association:
#app/models/message.rb
has_many :images, -> { select("#{Image.table_name}.*", "#{ImageMessage.table_name}.caption AS caption") }, class_name: 'Image', through: :image_messages, dependent: :destroy




Delegate

We've just found the delegate method, which does exactly what this needs. However, it only seems to work for has_one and belongs_to associations

We just got this working with a single association, but it seems it does not work for collections (just takes you to a public method)



Question

Do you know any way we could return the .caption attribute from the ImageMessage join model through the Image model?

We have this currently:
#app/models/image.rb
Class Message < ActiveRecord::Base
    has_many :image_messages
    has_many :messages, through: :image_messages

    delegate :caption, to: :image_messages, allow_nil: true
end

#app/models/image_message.rb
Class ImageMessage < ActiveRecord::Base
    belongs_to :image
    belongs_to :message

    def self.caption # -> only works with class method
        #what do we put here?
    end
end




Update

Thanks to Billy Chan (for the instance method idea), we have got it working very tentatively:
#app/models/image.rb
Class Image < ActiveRecord::Base
    #Caption
    def caption
        self.image_messages.to_a
    end
end

#app/views/messages/show.html.erb
<%= @message.images.each_with_index do |i, index| %>
    <%= i.caption[index][:caption] %> #-> works, but super sketchy
<% end %>
Any way to refactor, specifically to get it so that each time .caption is called, it returns the image_message.caption value for that particular record? 
解决方案
delegate is just a shorthand as equivalent instance method. It's not a solution for all, and there are even some debate that it's not so explicit. 

You can use an instance method when simple delegate can't fit.


    def image_message_caption
      image_messages.class.caption
    end


I reviewed and found any association is unnecessary is this case. The ImageMessage's class method caption is more like a constant, you can refer it directly.
def image_message_caption
  ImageMessage.caption
end


                        
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