xml - How do I get Nokogiri to understand my namespaces?

Question

I have the following XML document:

<samlp:LogoutRequest ID="123456789" Version="2.0" IssueInstant="200904051217">
  <saml:NameID>@NOT_USED@</saml:NameID>
  <samlp:SessionIndex>abcdefg</samlp:SessionIndex>
</samlp:LogoutRequest>

I'd like to get the content of the SessionIndex (that is, 'abcdefg') out of it. I've tried this:

XPATH_QUERY = "LogoutRequest[@ID][@Version='2.0'][IssueInstant]/SessionIndex"
SAML_XMLNS  = 'urn:oasis:names:tc:SAML:2.0:assertion'
SAMLP_XMLNS = 'urn:oasis:names:tc:SAML:2.0:protocol'

require 'nokogiri'
doc = Nokogiri::XML(xml)
doc.xpath(XPATH_QUERY, 'saml' => SAML_XMLNS, 'samlp' => SAMLP_XMLNS)

but I get the following errors:

Nokogiri::XML::SyntaxError: Namespace prefix samlp on LogoutRequest is not defined
Nokogiri::XML::SyntaxError: Namespace prefix saml on NameID is not defined
Nokogiri::XML::SyntaxError: Namespace prefix samlp on SessionIndex is not defined

I've tried adding the namespaces to the XPath query, but that doesn't change anything.

Why can't I convince Nokogiri that the namespaces are valid?

Answer 1

I see a two different options for you:

1. Remove all the namespaces

   http://www.rubydoc.info/github/sparklemotion/nokogiri/Nokogiri/XML/Document#remove_namespaces%21-instance_method

   Brute force way of doing it. Could lead to problems where there are namespace collisions.

2. Use collect_namespaces

   http://www.rubydoc.info/github/sparklemotion/nokogiri/Nokogiri/XML/Document#collect_namespaces-instance_method

   A much better solution. You could use this once to identify the namespaces (say in irb) and hard-code them.

   OR

   Use it at runtime, and supply it as the second argument to https://www.rubydoc.info/github/sparklemotion/nokogiri/Nokogiri/XML/Searchable#xpath-instance_method