gistfile1.rb

class Tag < ActiveRecord::Base
  has_many :taggings
end
 
class Tagging < ActiveRecord::Base
  set_inheritance_column nil
  belongs_to :tag
  belongs_to :posts
end

class ImportantTagging < Tagging
  # uses a string 'flag' column but could be changed to a boolean important
  # column later, hence the need to encapsulate!
  default_scope :conditions => { :flag => 'important' }
end
 
class Post < ActiveRecord::Base
  has_many :taggings
  has_many :important_taggings
  has_many :tags, :through => :taggings, :source => :tag
  has_many :important_tags, :through => :important_taggings, :source => :tag
end