How can I get nokogiri to select node attributes and add them to other nodes?

Question

Is it possible to grab a following element's attributes and use them in the preceding one like this?:

<title>Section X</title>
<paragraph number="1">Stuff</paragraph>
<title>Section Y</title>
<paragraph number="2">Stuff</paragraph>

into:

<title id="ID1">1. Section X</title>
<paragraph number="1">Stuff</paragraph>
<title id="ID2">2. Section Y</title>
<paragraph number="2">Stuff</paragraph>

I have something like this but get nodeset or string errors:

frag = Nokogiri::XML(File.open("test.xml"))

frag.css('title').each { |text| 
text.set_attribute('id', "ID" + frag.css("title > paragraph['number']"))}

Answer 1

next_sibling should do the job

require 'rubygems'
require 'nokogiri'

frag = Nokogiri::XML(DATA)
frag.css('title').each { |t| t['id'] = "ID#{t.next_sibling.next_sibling['number']}" }
puts frag.to_xml

__END__
<root>
<title>Section X</title>
<paragraph number="1">Stuff</paragraph>
<title>Section Y</title>
<paragraph number="2">Stuff</paragraph>
</root>

Because whitespace is also a node, you have to call next_sibling twice. Maybe there is a way to avoid this.

Alternatively you can use an xpath expression to select the number attribute of the next paragraph

t['id'] = "ID#{t.xpath('following-sibling::paragraph/@number').first}"