00_xml_builder.rb

require "nokogiri"

some_data = nil

builder = Nokogiri::XML::Builder.new encoding: "UTF-8" do |xml|
  xml.root do
    # insert <options> first, and keep
    # a reference to this node
    options_node = xml.options


    # build more XML and do stuff here AND collect some data
    xml.some_node do
      some_data = ["foo", "bar"]
    end

    # use data here, to create child nodes on
    # "options"
    # Without the ".foo" I just get a method missing error:
    # undefined method `options_node' for main:Object (NoMethodError)
    options_node.foo do |bar|
      some_data.each do |data|
        bar.child attr: data
      end
    end
  end
end

puts builder.to_xml

01_desired_result.xml

<?xml version="1.0" encoding="UTF-8"?>
<root>
  <options>
    <child attr="foo"/>
    <child attr="bar"/>
  </options>
  <some_node/>
</root>

02_current_result.xml

<?xml version="1.0" encoding="UTF-8"?>
<root>
  <options class="foo">
    <child attr="foo"/>
    <child attr="bar"/>
  </options>
  <some_node/>
</root>