why is this wrong

not_right.rb

require "nokogiri"

xml = Nokogiri::XML(File.open("some_shit.xml"))

xml.at_css("xmldata")
# => nil

xml.xpath("//soap12:Envelope")
# => works, returns element and children

xml.xpath("//soap12:Envelope//soap12:Body")
# => works, returns element and children

xml.xpath("//soap12:Envelope//soap12:Body//postdata")
# => suddenly does not work, won't detect the <postdata> element

some_shit.xml

<?xml version="1.0" encoding="utf-8" ?>
<soap12:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap12="http://www.w3.org/2003/05/soap-envelope">
  <soap12:Header>
    <cAuthentication xmlns="http://ar.masstech-pts.org/">
      <UserName>username</UserName>
      <Password>password1</Password>
    </cAuthentication>
  </soap12:Header>
  <soap12:Body>
    <postdata xmlns="http://ar.masstech-pts.org/">
      <xmldata></xmldata>
    </postdata>
  </soap12:Body>
</soap12:Envelope>

it is caused by postdata's special namespace. use <postdata> and both the css and xpath statements work.

When I do xml.xpath("//<postdata>") or xml.at_css("<postdata>"), I get a Nokogiri::XML::XPath::SyntaxError because of the angle brackets

the original query is correct, im saying the element 'postdata' in the XML is defining its own namespace which is putting it out of reach of the normal query. i dont understand xml namespaces that well but if postdata has to have a namespace, perhaps use a namespace prefix, such as how soap12 works.