Question

The following examples differ only in the Book and Movie models.

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

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

What does this difference mean?

---

Example 1:

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

Are you sure the 2nd example works? I don’t have an rails environment in windows.
For my understanding about your question,

case1.

has_many :taggings, :as => :taggable

here the “taggable” is not an exact Model name, it’s just an alias. (there’s no Taggable class)

case2.

has_many :taggings, :through => :taggable

here the “taggable” must be an real (exsting) model, I mean there must be an Taggable class somewhere.

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