XML : Nokogiri build xml with conditions

So.. I wanna build a xml example, but sometimes I need put xml over some condition, something like that:

  require 'nokogiri'    def add_xml(xml)    xml.send(:level, 'last')  end    builder = Nokogiri::XML::Builder.new do |xml|    xml.send(:level_1) {      xml.send(:level_2) {        add_xml(xml) if condition      }    }  end    puts builder.to_xml    

The problem is that my condition is false, so my block level_2 break, like below:

  <?xml version="1.0"?>  <level_1>    <level_2/>  </level_1>    

Someone have some idea about what can I do about that?