如何避免重复的has_many：通过关系？ [英] how to avoid duplicates in a has_many :through relationship?

问题描述

如何实现以下功能？我有两个模型（博客和读者）和一个JOIN表，将允许我们之间有一个N：M的关系：

  class Blog< ActiveRecord :: Base 
 has_many：blogs_readers，：dependent => ：destroy 
 has_many：reader，：through => ：blogs_readers 
 end 
 
 class Reader< ActiveRecord :: Base 
 has_many：blogs_readers，：dependent => ：destroy 
 has_many：blogs，：through => ：blogs_readers 
 end 
 
 class BlogsReaders< ActiveRecord :: Base 
 belongs_to：blog 
 belongs_to：reader 
 end 
  

我现在要做的是将读者添加到不同的博客。但是，条件是，我只能在一个博客上添加一个读者。所以在 BlogsReaders中不能有任何重复（相同的 readerID ，相同的 blogID ） 表。第二个问题是，如何获取读者未订阅的博客列表（例如，填写下拉菜单）

解决方案

如何：

  Blog.find（：all，
：conditions => ['id NOT IN（？）'，the_reader.blog_ids] ）
  

Rails通过关联方法来处理我们的ids集合！ ：）

http： //api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html

How can I achieve the following? I have two models (blogs and readers) and a JOIN table that will allow me to have an N:M relationship between them:

class Blog < ActiveRecord::Base
  has_many :blogs_readers, :dependent => :destroy
  has_many :readers, :through => :blogs_readers
end

class Reader < ActiveRecord::Base
  has_many :blogs_readers, :dependent => :destroy
  has_many :blogs, :through => :blogs_readers
end

class BlogsReaders < ActiveRecord::Base
  belongs_to :blog
  belongs_to :reader
end

What I want to do now, is add readers to different blogs. The condition, though, is that I can only add a reader to a blog ONCE. So there mustn't be any duplicates (same readerID, same blogID) in the BlogsReaders table. How can I achieve this?

The second question is, how do I get a list of blog that the readers isn't subscribed to already (e.g. to fill a drop-down select list, which can then be used to add the reader to another blog)?

解决方案

What about:

Blog.find(:all,
          :conditions => ['id NOT IN (?)', the_reader.blog_ids])

Rails takes care of the collection of ids for us with association methods! :)

http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html

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