How to use xmlns declarations with XPath in Nokogiri

That XPath query looks for elements that are not in any namespace. You need to tell your XPath processor that you are looking for elements in the http://sdb.amazonaws.com/doc/2007-11-07/ namespace.

One way to do that with Nokogiri is:

doc = Nokogiri::XML.parse(...)
doc.xpath("//aws:Item/aws:Attribute[Name='Foo']/aws:Value", {"aws" => "http://sdb.amazonaws.com/doc/2007-11-07/"})

I found "Namespaces in XML" really helpful in understanding what's going on.

Basically if you have a namespace defined via xmlns=, you must use a namespace in your XPath searches.

So in your case, you could do one of three things:

• Remove the xmlns attribute from the root SearchResponse. In that case your original, namespace-less XPath query will work.

• Use the default namespace in your XPath query:

  doc/"//xmlns:Item/xmlns:Attribute[xmlns:Name='Foo']/xmlns:Value"
  

• Define a custom namespace in the second argument of the xpath method and use that in your query, as shown in hrnt's solution above.