Taxonomy
this gem is an active record plugin. think of it as a merger of rubygems.org/gems/acts-as-taggable-on and rubygems.org/gems/awesome_nested_set because, well it is. With some glue code and updates to the models.
Setup Rails 3.x
Add to Gemfile:
gem 'taxonomy'
Run migration:
rails generate taxonomy:migration
rake db:migrate
Usage
to setup plain tags and a treed set of categories
has_taxonomy_on :tags, {:treed => [:categories]}
Tag.roots.where(:context => "category")
Tag.find_context_with_slug!("category", params[:slug])