Modified to handle tree with node attributes

hash_to_xml_example.rb

require 'nokogiri'

tree = {
  node_type: 'form',
  children: [
    {
      node_type: 'div',
      attrs: {
        class: 'awesome'
      },
      children: [
        {
          node_type: 'section',
          attrs: {
            class: 'nested'
          }
        }
      ]
    }
  ]
}

def xml_node(node, xml)
  node_type = node[:node_type]
  attrs = node[:attrs] || {}
  children = node[:children] || []

  xml.send(node_type, attrs) do
    children.each do |child|
      xml_node(child, xml)
    end
  end
end

builder = Nokogiri::XML::Builder.new do |xml|
  xml_node tree, xml
end

puts builder.to_xml save_with: 2