Nokogiri's Builder and nodes like <id> and <type>.rb

builder = Nokogiri::XML::Builder.new do |xml|
  xml.listings {
    xml.language 'en'

    properties.each do |property|

      xml.listing {
        # Instantiate a Nokogiri::XML::Node and insert it manually.
        node = Nokogiri::XML::Node.new('id', doc) # doc is actually Nokogiri::XML::Builder#doc.
        node.content = property.id
        insert(node)

        xml.name property.name
      }

    end
  }
end