Xml filtering in Lua

["Thomas A. Schmitz via ntg-context" <ntg-context@ntg.nl>]

Hi all,

I thought this was easy, but I overestimated my competence… I want to filter xml elements via their attributes and retrieve and typeset parts belonging together. Here is a small test file that explains what I’m trying:

\startbuffer[test]
<document>
   <topics>
      <topic id="test1">
	<title>This is the first test</title>
	<date>22/11/16</date>
      </topic>
      <topic id="test2">
	<title>This is the second test</title>
	<date>22/11/17</date>
      </topic>
   </topics>
   <chapters>
      <chapter id="test1">
	<content>
	  This will be the content of the <emph>first</emph> chapter.
	</content>
      </chapter>
      <chapter id="test2">
	 <content>
	  This will be the content of the <emph>second</emph> chapter.
	 </content>
      </chapter>
   </chapters>
</document>
\stopbuffer

\startxmlsetups xml:testsetups
        \xmlsetsetup{#1}{*}{-}
	\xmlsetsetup{#1}{document|chapters|chapter|content|emph}{xml:*}
\stopxmlsetups

\xmlregistersetup{xml:testsetups}

\startxmlsetups xml:document
	\xmlflush{#1}
\stopxmlsetups

\startxmlsetups xml:chapters
	\xmlflush{#1}
\stopxmlsetups

\startxmlsetups xml:chapter
	\xmlfunction {#1} {chapter}
	 \xmlflush {#1}	
\stopxmlsetups

\startxmlsetups xml:chapter:content
	\xmltext {#1} {content}
\stopxmlsetups

\startxmlsetups xml:emph
   {\em \xmlflush {#1}}
\stopxmlsetups

\startluacode
function xml.functions.chapter (t)
	 local ch_id = t.at.id
		local metadata = xml.filter (root, '../../topics/topic[@id=="%s"]', ch_id)
		print (inspect(metadata))
		lxml.command(t, ".", "xml:chapter:content")
		context.par ()
		context (ch_id)
		context.par ()
end
\stopluacode

\starttext
	\xmlprocessbuffer{main}{test}{}
\stoptext

The line with xml.filter does not work as I expected. How can I walk the tree, find the “topic” element with the same “id” attribute as the chapter I’m currently in, and then typeset the different children of the topic element? 

Thanks a lot and all best

Thomas

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