Reverse Polymorphic Associations An email digest can contain either articles or questions, called as @email_digest.digest_elements

reverse polymorphic.rb

class EmailDigest < ActiveRecord::Base
  has_many :digest_elements
  has_many :articles, :through => :digest_elements, :source => :element, :source_type => 'Article'
  has_many :questions, :through => :digest_elements, :source => :element, :source_type => 'Question'
end
 
class Article < ActiveRecord::Base
  has_one :article_element, :as =>:element
  has_one :email_digest, :through => :digest_elements
end

class Question < ActiveRecord::Base
  has_one :article_element, :as =>:element
  has_one :email_digest, :through => :digest_elements 
end

class DigestElement < ActiveRecord::Base
  belongs_to :email_digest
  belongs_to :element, :polymorphic => true
end

d = EmailDigest.new
d.digest_elements  # []
a = Article.new
d.save
d.digest_elements.create(:element => a)
d.digest_elements 
d.articles 

z20130917174839_create_digest_elements.rb

class CreateDigestElements < ActiveRecord::Migration
  def change
    create_table :digest_elements do |t|
      t.integer :email_digest_id
      t.integer :element_id
      t.string :element_type

      t.timestamps
    end
  end
end