gistfile1.rb

# just using ruby (see below) makes for a pretty easy fix in this case, but it could
# get pretty ugly pretty quick which a real world case.  Having a 'One True Namespace'
# for conditions would keep the ugly at bay.
#
# So, I think I like the idea of using named_scope as a namespace for conditions (ie
# Luke's idea)

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!
  ImportantConditions = {:flag => 'important'}

  named_scope :important, :conditions => ImportantConditions
end

class Post
  has_many :taggings
  has_many :tags, :through => :taggings, :source => :tag
  has_many :important_tags, :through => :taggings, :source => :tag, :conditions => Tagging::ImportantConditions
end