如何避免has_many:through关系中的重复？

Question

我怎样才能实现以下目标？我有两个模型(博客和读者)和一个JOIN表,它允许我在它们之间建立N:M关系:

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

我现在想做的是将读者添加到不同的博客中.但是,条件是我只能将博客添加到博客中.因此表中不得有任何重复(相同readerID,相同blogID)BlogsReaders.我怎样才能做到这一点？

第二个问题是,如何获得读者尚未订阅的博客列表(例如,填写下拉选择列表,然后可以将读者添加到另一个博客)？

Answer 1

Rails中内置的更简单的解决方案:

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

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

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

请注意将该:uniq => true选项添加到has_many呼叫中.

此外,您可能需要has_and_belongs_to_many在Blog和Reader之间考虑,除非您在连接模型上有一些其他属性(当前没有这些属性).那种方法也有一个:uniq选择.

请注意,这不会阻止您在表中创建条目,但它确实在您查询集合时只能获得每个对象中的一个.

更新

在Rails 4中,实现它的方法是通过范围块.以上更改为.

class Blog < ActiveRecord::Base
 has_many :blogs_readers, dependent:  :destroy
 has_many :readers,  -> { uniq }, through: :blogs_readers
end

class Reader < ActiveRecord::Base
 has_many :blogs_readers, dependent: :destroy
 has_many :blogs, -> { uniq }, through: :blogs_readers
end

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

Rails的更新5

使用uniq在范围块将导致错误 NoMethodError: undefined method 'extensions' for []:Array.distinct改为使用:

class Blog < ActiveRecord::Base
 has_many :blogs_readers, dependent:  :destroy
 has_many :readers,  -> { distinct }, through: :blogs_readers
end

class Reader < ActiveRecord::Base
 has_many :blogs_readers, dependent: :destroy
 has_many :blogs, -> { distinct }, through: :blogs_readers
end

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

Answer 2

这应该照顾你的第一个问题:

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

  validates_uniqueness_of :reader_id, :scope => :blog_id
end

Answer 3

Rails 5.1方式

class Blog < ActiveRecord::Base
 has_many :blogs_readers, dependent:  :destroy
 has_many :readers,  -> { distinct }, through: :blogs_readers
end

class Reader < ActiveRecord::Base
 has_many :blogs_readers, dependent: :destroy
 has_many :blogs, -> { distinct }, through: :blogs_readers
end

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

Answer 4

关于什么:

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

Rails通过关联方法为我们收集ids!:)

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