Question

以下示例仅在图书和电影模型中有所不同。

示例1：Book has_many :taggings, :as => :taggable

示例2：Book has_many :taggings, :through => :taggable

这种差异意味着什么？

class Book < ActiveRecord::Base
  has_many :taggings, :as => :taggable
  has_many :tags, :through => :taggings
end

class Movie < ActiveRecord::Base
  has_many :taggings, :as => :taggable
  has_many :tags, :through => :taggings
end

class Tag < ActiveRecord::Base
  has_many :taggings
  has_many :books, :through => :taggings, :source => :taggable, :source_type => "Book"
  has_many :movies, :through => :taggings, :source => :taggable, :source_type => "Movie"
end

class Tagging < ActiveRecord::Base
  belongs_to :taggable, :polymorphic => true
  belongs_to :tag
end

Example 2:

class Book < ActiveRecord::Base
  has_many :taggings, :through => :taggable
  has_many :tags, :through => :taggings
end

class Movie < ActiveRecord::Base
  has_many :taggings, :through => :taggable
  has_many :tags, :through => :taggings
end

class Tag < ActiveRecord::Base
  has_many :taggings
  has_many :books, :through => :taggings, :source => :taggable, :source_type => "Book"
  has_many :movies, :through => :taggings, :source => :taggable, :source_type => "Movie"
end


class Tagging < ActiveRecord::Base
  belongs_to :taggable, :polymorphic => true
  belongs_to :tag
end

Answer 1

你确定第二个例子有效吗？我在Windows中没有rails环境。为了解我的问题，

情形1。

has_many :taggings, :as => :taggable

这里“taggable”不是一个确切的Model名称，它只是一个别名。（没有Taggable类）

情况2。

has_many :taggings, :through => :taggable

这里“taggable”必须是一个真实的（exsting）模型，我的意思是某处必须有一个Taggable类。

参考：http://guides.rubyonrails.org/association_basics.html#polymorphic-associations

Answer 2

假设这些例子工作..而且taggable是一个真实的模型......正如Siwei建议的那样可能实际上并不起作用......

:as defines a polymorphic association

和

:through says that
   if A has many B's
   and B has many C's
   then A has many C's through B

多态多对多

2 个答案: