snippet.rb

Tag.class_eval do
  default_scope :joins => :taggings
  named_scope :popular, lambda {{
    :select => "#{Tag.quoted_table_name}.*, COUNT(*) AS popularity", 
    :group => "#{Tagging.quoted_table_name}.tag_id", 
    :order => "popularity DESC"
  }}
  named_scope :on, lambda {|klass| {
    :joins => "INNER JOIN #{klass.quoted_table_name} ON #{klass.quoted_table_name}.id = #{Tagging.quoted_table_name}.taggable_id " + \
              "AND #{Tagging.quoted_table_name}.taggable_type = '#{klass.to_s}'"
  }}
end