xslt - Use XPath to find nodes whose attribute is missing from another part of XML document -

i've got xml document similar this:

<pipeline>     <log>         <error stagename="one" message="ack!" />         <error stagename="two" message="ack!" />         <error stagename="three" message="ack!" />     </log>     <stages>         <stage name="one" />         <stage name="two" />     </stages> </pipeline> 

i'm trying use xslt make report of errors grouped stage. @ end of report, want errors not-associated stage. i'm able report on each stage's errors. need way conditionally show other errors.

this closest i've come:

<xsl:template match="/pipeline">     <xsl:variable name="unknown.stages"                    select="log/error[@stagename != stages/stage/@name]" />      <xsl:if test="count($unknown.stages) > 0">         <h1>other errors</h1>         <xsl:apply-templates select="$unknown.stages" />     </xsl:if> </xsl:template> 

but ends showing errors instead of errors not associated stage. there xpath query can use in xsl:variable declaration select error nodes stagename attribute has value doesn't exist in list of stages/stage elements?

update: here correct xpath query. have create key on stages element, , use key find errors don't match:

<xsl:key name="stagename" match="/pipeline/stages/stage" use="@name" />  <xsl:template match="/pipeline">     <xsl:variable name="unknown.stages"                    select="log/error[not(key('stagename',@stagename)]" />      <xsl:if test="count($unknown.stages) > 0">         <h1>other errors</h1>         <xsl:apply-templates select="$unknown.stages" />     </xsl:if> </xsl:template> 

are sure getting errors shown? current xpath looking stages element child of current error element selecting. have expected expression more this

<xsl:variable name="unknown.stages" select="log/error[@stagename != //stages/stage/@name]" /> 

having said that, returns error elements because return error elements there exists stage different name.

anyway, solve this, consider using key stage elements

<xsl:key name="stages" match="stage" use="@name" /> 

then can write expression this, should check existence require:

<xsl:variable name="unknown.stages"select="log/error[not(key('stages', @stagename))]" />