partial local

Category.rb

  def self.roots
    where(:parent_id => nil)
  end

layouts-application.html.haml

  %ul#menu= render 'shared/categories_menu', :locals => { :categories => Category.roots }

shared-_categories_menu.html.haml

- categories.each do |category|
  %li
    = link_to category.name, category
    - unless category.children.empty?
      %ul= render 'shared/categories_menu', :locals => { :categories => category.children }