使用libxml + ruby​​的xpath搜索

Question

I am trying to search for a specific node in an XML file using XPath. This search worked just fine under REXML but REXML was too slow for large XML docs. So moved over to LibXML .

My simple example is processing a Yum repomd.xml file, an example can be found here: http://mirror.san.fastserv.com/pub/linux/centos/6/os/x86_64/repodata/repomd.xml

My test script is as follows:

require 'rubygems'
require 'libxml'

p = LibXML::XML::Parser.file( "/tmp/dr.xml")
repomd = p.parse

filelist = repomd.find_first("/repomd/data[@type='filelists']/location@href")
puts "Length: " + filelist.length.to_s
filelist.each do |f|
   puts f.attributes['href']
end

I get this error:

Error: Invalid expression.
/usr/lib/ruby/gems/1.8/gems/libxml-ruby-2.7.0/lib/libxml/document.rb:123:in `find': Error: Invalid expression. (LibXML::XML::Error)
from /usr/lib/ruby/gems/1.8/gems/libxml-ruby-2.7.0/lib/libxml/document.rb:123:in `find'
from /usr/lib/ruby/gems/1.8/gems/libxml-ruby-2.7.0/lib/libxml/document.rb:130:in `find_first'
from /tmp/scripty.rb:6

I have also tried simpler examples like below, but still no dice.

p = LibXML::XML::Parser.file( "/tmp/dr.xml")
repomd = p.parse
filelist = repomd.root.find(".//location")
puts "Length: " + filelist.length.to_s

In the above case I get the output:

Length: 0

Your inspired guidance would be greatly appreciated, and I have searched for what I am doing wrong, and I just can't figure it out...

Here is some code that will fetch the file and process it, still doesn't work...

require 'rubygems'
require 'open-uri'
require 'libxml'

raw_xml = open('http://mirror.san.fastserv.com/pub/linux/centos/6/os/x86_64/repodata/repomd.xml').read
p = LibXML::XML::Parser.string(raw_xml)
repomd = p.parse
filelist = repomd.find_first("//data[@type='filelists']/location[@href]")
puts "First: " + filelist

Answer 1

In the end I reverted back to REXML and used stream processing. Much faster and much easier XPath syntax implementation.

Answer 2

Looking at your code,it seems you want to collect only those location elements which has href attribute. If that's the case below should work:

"//data[@type='filelists']/location[@href]"