通過3個聯

Question

模型定義選擇從has_many_and_belongs_to_many對象：

class Article < ActiveRecord::Base 
    has_and_belongs_to_many :groups 
end 

class Group < ActiveRecord::Base 
    has_and_belongs_to_many :articles 
    has_many :domains, :inverse_of=>:group 
end 

class Domain < ActiveRecord::Base 
    belongs_to :group, :inverse_of=>:domains 
    has_many :pages, :inverse_of=>:domain 
end 

class Page < ActiveRecord::Base 
    belongs_to :domain, :inverse_of=>:pages 
    belongs_to :article, :inverse_of=>:pages 
end 

對於具體的Article我想選擇所有Domains沒有與該Article相關的任何Page（通過groups.domains相關）。

class Article < ActiveRecord::Base 
    has_and_belongs_to_many :groups 

    def avaiable_domains 
    groups.domains("where not exists page with article_id=#{id}")) ##???? 
    #I have code mentioned at the end of this question, but I don't want use SQL, just pure active query 
    end 
end 

是否有可能將它寫入純活動記錄查詢或Arel（在沒有SQL的情況下）？

現在我使用的是這樣的：

Domain.where(:group_id=>group_ids).where('NOT EXISTS(SELECT 1 FROM pages WHERE pages.domain_id=domains.id AND article_id=?)',id) 

Answer 1

Domain.joins(:groups => :articles, :pages).where("pages.article_id <> :id AND articles.id = :id", id: article_id) 

在文章模型

def available_domains 
    Domain.available(id) 
    end