gistfile1.rb

  def display_tree_navigation(tree, parent_id)
    ret = "<ul>"
    tree.each do |node|
      if node.parent_id == parent_id
        ret += "<li>"
        ret += link_to node.title, node
        ret += display_tree_navigation(node.children, node.id)
        ret += "</li>"
      end
    end
    ret += "</ul>"
  end