CODEWITHSUNDEEP
xpathxquerybasex
Krzysztof Czeronko
Krzysztof Czeronko

Reputation: 569

How to convert string to Xpath in Xquery function (BaseX)

I am writing Xquery function for BaseX which gets one of arguments as name of the element node. This name is then used in Xpath, but in general I cannot convert string to element.

This is how the method looks like

declare function prefix:getElementWithValue($root as document-node()?, $elem as xs:string?, $minVal as xs:float?, $maxVal as xs:float?)
as element()*

{ 
  let $e := element {$elem} {""}
  for $x in $root//SUBELEM
  return if ($x//$e/@ATTRIB>=$minVal and $x//$e/@ATTRIB<=$maxVal) then ($x)
};

and the call

return prefix:getElementWithValue($db, "SomeElem", 10.0, 10.0)

and I am getting empty response from that. If I replace the $x//$e with $x//SomeElem it returns proper response. From the QueryPlan I see that the $e is treated as literal value. XPATH is not $x//SomeElem/@ATTRIB but $x//$e/@ATTRIB

So my question is how to covert string to type that can be used in XPATH?

Upvotes: 0

Views: 586

Answers (1)

Michael Kay
Michael Kay

Reputation: 163262

XQuery does not have a standard function to evaluate a dynamically-constructed XPath expression.

Many XQuery processors offer some kind of extension function that does this, however. For example, BaseX offers query:eval():

https://docs.basex.org/wiki/XQuery_Module#xquery:eval

Note that variables in XQuery represent values, not fragments of expression text. Your expression $x//$e/@ATTRIB is equivalent to $x//"SomeElem"/@ATTRIB, which is quite different from $x//SomeElem/@ATTRIB.

If you know that $elem will always be an element name, then you can write $x//*[name()=$e]/@ATTRIB. But take care over namespaces.

Upvotes: 3

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