add_child_ns.rb

#!/usr/bin/env ruby

require 'nokogiri'

src =<<EOD
<wrapper xmlns="ns" xmlns:extra="extra">
    <record xml:id="r1">
        <field>aaa</field>
        <field extra:type="second">bbb</field>
    </record>
</wrapper>
EOD

src_doc = Nokogiri::XML(src)
record = src_doc.at('//base:record', {'base' => "ns"})

puts "== Original document"
puts src_doc.to_xml

dest_doc = Nokogiri::XML("<summary xmlns='summary'/>")
dest_doc.root.add_child(record.clone)

puts
puts "== New document"
puts "== The attribute `type` should be in namespace `extra`, but it is not"
puts dest_doc.to_xml