Achieven nested tags

environment.rb

config.gem "acts-as-taggable-on", :source => "http://gemcutter.org", :version => '2.0.0.rc1'

generate_migration.sh

./script/generate acts_as_taggable_on_migration

install_plugin.sh

./script/plugin install http://github.com/rails/acts_as_tree.git

migration.rb

class NestedTags < ActiveRecord::Migration
  def self.up
    add_column :tags, :parent_id, :integer
  end

  def self.down
    remove_column :tags, :parent_id
  end
end

nested_tags_migration.sh

./script/generate migration nested_tags

tag.rb

class Tag < ActiveRecord::Base
  acts_as_tree
end