Rails Polymorphic many-to-many

15 Aug 2013

Understand [:source] option of [has_one/has_many through]

Sometimes, you want to use different names for different associations. If the name you want to use for an association on the model is not the same as the association on the [:through] model, you can use [:source] option to specify it.

Here is an example: Assume we have three models: Pet, Dog and Dog::Breed


class Pet < ActiveRecord::Base
  has_many :dogs
end

class Dog < ActiveRecord::Base
  belongs_to :pet
  has_many :breeds
end

class Dog::Breed < ActiveRecord::Base
  belongs_to :dog
end

In this case, we have chosen to namespace the Dog::Breed, because we want to access Dog.find(123).breeds as a nice and convenient association. Now, if we want to create a has_many :dog_breeds, :through => :dogs association on Pet, we suddenly have a problem. Rails will not be able to find a :dog_breeds association on Dog, so Rails can not possibly know which Dog association you want to use. With :source, we are telling Rails to look for an association called :breeds on the Dog model, and use that.


class Pet < ActiveRecord::Base
  has_many :dogs
  has_many :dog_breeds, :through => :dogs, :source => :breeds
end

Understand :source_type option of Polymorphic

Assume we have models: Tag, Tagging, Book and Movie


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

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

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

From the above code, we know that :source_type option is used only when :source option can not specify the target model in polymorphic.

Reference


http://www.brentmc79.com/posts/polymorphic-many-to-many-associations-in-rails
http://stackoverflow.com/questions/9500922/need-help-to-understand-source-type-option-of-has-one-has-many-through-of-rails