class Tag
  has_many :taggings
  named_scope :important, :include => { :taggings => :important }
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
  has_many :taggings
  has_many :tags, :through => :taggings, :source => :tag
end
 
# post.tags.important