Gr8 post by Daniel Wanja for Use acts_as_nested_set with Flex.

ActiveRecord: app/models/category.rb

app/models/category.rb

class Category < ActiveRecord::Base
  acts_as_nested_set
end

Controller: app/controllers/categories_controller.rb

app/controllers/categories_controller.rb

class CategoriesController < ApplicationController
  def index
     Category.result_to_attributes_xml(Category.root.full_set)

  end
end

Flex Application: ActsAsNestedSet.mxml

ActsAsNestedSet.mxml

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
    layout="vertical"
    applicationComplete="categories.send()">
<mx:HTTPService id="categories" url="http://localhost:3000/categories" resultFormat="e4x" />
<mx:Tree dataProvider="{categories.lastResult}"
    labelField="@name"
    width="100%" height="100%" />
</mx:Application>

Result:

XML generated by Category.result_to_attributes_xml(Category.root.full_set):

XML generated by Category.result_to_attributes_xml(Category.root.full_set)

<node name="Main Category" id="15" description="">

  <node name="Cameras & Photo" id="16" description="">

    <node name="Bags" id="17" description=""/>
    <node name="Accessories" id="18" description=""/>

    <node name="Analog Cameras" id="19" description=""/>
    <node name="Digital Cameras" id="20" description=""/>

  </node>
  <node name="Cell Phones" id="21" description="">

    <node name="Accessories" id="22" description=""/>
    <node name="Phones" id="23" description=""/>

    <node name="Prepaid Cards" id="24" description=""/>
  </node>

  <node name="Dvds" id="25" description="">
    <node name="Blueray" id="26" description=""/>

    <node name="HD DVD" id="27" description=""/>
    <node name="DVD" id="28" description=""/>

  </node>
</node>

I used the http://wiki.rubyonrails.org/rails/pages/BetterNestedSet plugin.

Too cool!