to_xml_example.rb

gem 'activemodel'

require 'active_model'

module Serializable
  include ActiveModel::Serializers::Xml

  def self.included(base)
    base.extend ClassMethods
    base.extend ActiveModel::Naming
  end

  def initialize( attrs = {} )
    attrs.each do |key,value|
      send("#{key}=", value)
    end
  end

  module ClassMethods

    def define_attributes( *keys )
      attr_accessor *keys

      define_method("attributes") do
        values = {}
        keys.each do |key|
          values[key] = send(key)
        end
        values
      end
    end

  end

end

class User
  include Serializable
  define_attributes :name, :products, :company
end

class Product
  include Serializable
  define_attributes :name, :price
end

class Company
  include Serializable
  define_attributes :name
end

company = Company.new( :name => 'Some Company' )

products = [
    Product.new(:name => 'First one', :price => 30),
    Product.new(:name => 'Second one', :price => 20)
]

user = User.new( :products => products, :company => company, :name => 'Some User' )

puts user.to_xml( :methods => [ :products, :company, :name ] )

user.xml

<?xml version="1.0" encoding="UTF-8"?>
<user>
  <company>
    <name>Some Company</name>
  </company>
  <name>Some User</name>
  <products type="array">
    <product>
      <name>First one</name>
      <price type="integer">30</price>
    </product>
    <product>
      <name>Second one</name>
      <price type="integer">20</price>
    </product>
  </products>
</user>