Implementing current()

[Hipska]

I'm wondering why the current() function hasn't been implemented, and if it would make much time to do so.

I'm currently in the situation where I would need a query like //cd[@title=current()/@ref]. The node navigator used for the expression evaluate isn't the document root, but a previously selected node.

[Hipska]

@zhengchun?

[zhengchun]

current() is only supported by XSLT. You can try this //cd[@title=@ref]

[Hipska]

Doesn't that only select cd nodes of which title attribute equals their ref attribute?

Edit: wow that actually worked.. Thanks man!

[Hipska]

Scrap that, it seems it didn't work, it always picks the first one in this case: ../../measType[@p=@p] (as I would expect)

[zhengchun]

Hello, Need a sample document to test.

or try this ../../measType[@p]

[Hipska]

That gives the same result. Its strange, as I was really under the impression your solution first worked..

Anyway, this is a simplified version of the file:

<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="MeasDataCollection.xsl"?>
<measCollecFile xmlns="http://www.3gpp.org/ftp/specs/archive/32_series/32.435#measCollec">
	<measData>
		<managedElement localDn="foobar.example.com" />
		<measInfo measInfoId="metric_name">
			<measType p="1">field1</measType>
			<measType p="2">field2</measType>
			<measValue measObjLdn="identifier">
				<r p="1">31854</r>
				<r p="2">159773</r>
			</measValue>
		</measInfo>
		<measInfo measInfoId="metric_name2">
			<measType p="1">field3</measType>
			<measType p="2">field4</measType>
			<measValue measObjLdn="identifier">
				<r p="1">1234</r>
				<r p="2">567</r>
			</measValue>
		</measInfo>
	</measData>
</measCollecFile>

And the selected node is each time r, so I need an xpath to find out the field name of it.

[zhengchun]

like this //r[@p]/ancestor::measInfo/measType[@p]

//r[@p]/../../measType[@p]

[Hipska]

That's even worse, now all metrics have field name field1.

The Path is run on each r node. So I think only ../../measType[@p=current()/@p] would work..

[zhengchun]

using the nested query to solve. like this

for _,n:=range xmlquery.Find(doc,"//r[@p]")
{
    for _,nn:=range xmlquery.Find(n,`../../measType[@p='`+n.SelectAttr("p")+`']`){}
}

[Hipska]

Well, yeah. I can't change the code. I'm using telegraf: https://github.com/influxdata/telegraf/blob/6dc2b99549b3fc4684295ebcd2975cee96a1fa28/plugins/parsers/xpath/parser.go#L419-L447

[zhengchun]

I tested //r[@p=../../measType/@p] and it only return the first one node(expected matched 4 node but only gave 2), this is a bug.

[Hipska]

Indeed, see GO playground vs XPather. Can I help with trying to fix it?

Edit: And what should be the correct path be after the fix? ../../measType[@p] or ../../measType[@p=@p]?

[zhengchun]

//r[@p=../../measType/@p] should be your expression to sove your problem.

[Hipska]

No, I have an r and need an xpath to find it’s measType.

Your xpath selects every r that has a corresponding measType.

[Hipska]

@zhengchun any update on this? Can I help to fix this?

[zhengchun]

@Hipska , You can try to fix this and submit PR.

[Hipska]

I would love to fix it, but I don't see where the bug is.

[zhengchun]

Sorry,I no idea how to implement Current() now, It's a XSLT function.

[Hipska]

Oh, but it seems you fixed it?

[zhengchun]

I just fixed this expression a[@b=../@b], fixed bug with parent query on current node in the filter query.

[Hipska]

Hmm, then this playground should return 4 nodes, right? https://go.dev/play/p/qeMkl0NqoOT

Edit: nevermind, there is no release yet 🙂 https://go.dev/play/p/sI48ALhbizY 👍

[Hipska]

I just tested latest version of telegraf (which includes the latest version of this library), but my use-case seems still not to be solved. I'm also not sure what the correct xpath for my problem should be.

Edit: See the following https://go.dev/play/p/ozvF3CfdDXT
I would expect this to be the output of that:

#0 <r p="1">31854</r>
	<measType p="1">field1</measType>
#1 <r p="2">159773</r>
	<measType p="2">field2</measType>
#2 <r p="1">1234</r>
	<measType p="1">field3</measType>
#3 <r p="2">567</r>
	<measType p="2">field4</measType>

Program exited.

Do You agree? If not, what should my second xpath query be to achieve this wanted result?

[zhengchun]

@Hipska , The actually result from your go code execute is correct, your expect result is not right.

	for i, n := range xmlquery.Find(doc, "//r[@p=../../measType/@p]") {
		fmt.Printf("#%d %s\n", i, n.OutputXML(true))
		for _, nn := range xmlquery.Find(n, `../../measType[@p=./@p]`) {  -- Here
			fmt.Printf("\t%s\n", nn.OutputXML(true))
		}
	}

xmlquery.Find(n, ../../measType[@p=./@p]), HERE, you tell xpath to start search from the specified node n, xpath will according on your ../../measType[@p=./@p] to matching. and @p=./@p just telling xpath target node have @p attribute, not your expect `@p=n[@p]'.

You should change your query from xmlquery.Find(n, "../../measType[@p=./@p]") to xmlquery.Find(n, "../../measType[@p=“+ n.Attr("p") +”]“)

You can try take your xpath expression execute on the another program language and see the result.

[Hipska]

@zhengchun sorry for this late response. the code itself cannot be changed as this is what telegraf is doing. So I am looking for the correct xpath query to achieve my wanted result..

[zhengchun]

So, what is your expected xpath if we can implement the current() function.

In the above example, if you can nested for...loop why you can't change xpath expression to replace ../../measType[@p=./@p] with ../../measType[@p=“+ n.Attr("p") +”]?

You can provide an example and what is you expected result.

[Hipska]

Hi, sorry to be so late on this..

In the telegraf config, I'm only able to pass a string, see earlier shared code. So I need a single string/xpath that returns the expected measType node. ../../measType[@p=“+ n.Attr("p") +”] is not possible and will not work.

[zhengchun]

I find a solution that saving the query node 'n' as a local variable used
by current() function in the query context, like this jsonquery.Find(n,"../../measType[@p=current()/@p]")

[Hipska]

Yeah, the whole reason of this issue is because of the current() function is not implemented:

panic: not yet support this function current()

[Hipska]

@zhengchun can you point me to where I could implement that?

[zhengchun]

Implementing current() is a big work, you need to save the current matched node and then used by current() function. One possible approach is to use context package to save the current node.

[Hipska]

Okay, but where could I start implementing it?

[zhengchun]

1. Add a new current() function:

   xpath/build.go

   Line 335 in 14e235f

   func (b *builder) processFunction(root *functionNode, props *builderProp) (query, error) {

2. Save the current node in query interface, like child::*, descendant::*, parent::*...:
   

   xpath/query.go

   Line 53 in 14e235f

   type query interface {

These are only my guesses and cannot be guaranteed to be right.