Nokogiri/Xpath namespace query

Question

I'm trying to pull out the dc:title element using an xpath. I can pull out the metadata using the following code.

doc = <<END
<?xml version="1.0" encoding="UTF-8"?>
<package xmlns="http://www.idpf.org/2007/opf" version="2.0">
  <metadata xmlns:dc="URI">
    <dc:title>title text</dc:title>
  </metadata>
</package>
END

doc = Nokogiri::XML(doc)

# Awesome this works!
puts '//xmlns:metadata'
puts doc.xpath('//xmlns:metadata')
# => <metadata xmlns:dc="URI"><dc:title>title text</dc:title></metadata>

As you can see the above appears to work correctly. However I don't seem to be able to get the title information from this node tree, all of the below fail.

puts doc.xpath('//xmlns:metadata/title')
# => nil

puts doc.xpath('//xmlns:metadata/dc:title')
# => ERROR: `evaluate': Undefined namespace prefix

puts doc.xpath('//xmlns:dc:title')
# => ERROR: 'evaluate': Invalid expression: //xmlns:dc:title

Could someone please explain how namespaces should be used in an xpath with the above xml doc.

Solution

All namespaces need to be registered when parsing. Nokogiri automatically registers namespaces on the root node. Any namespaces that are not on the root node you have to register yourself. This should work:

puts doc.xpath('//dc:title', 'dc' => "URI")

Alternately, you can remove namespaces altogether. Only do this if you are certain there will be no conflicting node names.

doc.remove_namespaces!
puts doc.xpath('//title')

Comments

[Simon Lepkin]

Using remove_namespace! is the most sensible thing to try first. But beware: if you're modifying this XML and submitting it to an external API, the API will (often) reject it without the namespaces.