gistfile1.rb

  def self.find_popular(args = {}) 
     find(:all, :select => 'tags.*, count(*) as popularity', 
       :limit => args[:limit] || 10,
       :joins => "JOIN taggings ON taggings.tag_id = tags.id JOIN things on taggings.taggable_id = things.id",
       :conditions => args[:conditions],
       :group => "taggings.tag_id", 
       :order => "popularity DESC"  ) 
   end