gistfile1.rb

class Tag
  has_many :taggings
end

class Tagging
  belongs_to :tag
  belongs_to :posts
  
  # uses a string 'flag' column but could be changed to a boolean important 
  # column later, hence the need to encapsulate!
  named_scope :important, :conditions => {:flag => 'important'}
end

class Post
  # Explicitly scope the association, preserving the encapsulation of "important"
  has_many :important_taggings, :scope => :important, :class_name => 'Tagging'
  has_many :tags, :through => :taggings, :source => :tag
  # Implicitly scope the association
  has_many :important_tags, :through => :important_taggings, :source => :tag
end