polymorphic.rb

# First, the new join table, where 'taggable' establishes the polymorphic association. 
class Tagging < ActiveRecord::Base #the join table
  belongs_to :tag
  belongs_to :taggable, :polymorphic => true #this line sets up polymorphism
end

# The tag model is associated with newspapers AND articles (polymorphism) via the join table. 
class Tag < ActiveRecord::Base  
  has_many :newspapers, :through => :taggings, :source => :taggable, 
           :source_type => "Newspaper" # informs rails how to get to newspapers from tags
  has_many :articles, :through => :taggings, :source => :taggable, 
           :source_type => "Article" # informs rails how to get to articles from tags
  has_many :taggings
end

# Article and Newspaper models are connected to tags instead of to eachother. 
Class Article < ActiveRecord::Base
  has_many :tags, :through => :taggings #gets tags via the taggings join table
  has_many :taggings, :through => :taggable # makes articles taggable
end

Class Newspaper < ActiveRecord::Base
  has_many :tags, :through => :taggings # gets tags via the taggings join table
  has_many :taggings, :through => :taggable # makes newspapers taggable
end