xslt node-set use in key function - find children by parent attribute

Question

I have an XML File.

Answer 1

Although this is not simple, I think you are making it much more complicated than it needs to be. Try the following stylesheet (not tested very thoroughly):

XSLT 1.0

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:exsl="http://exslt.org/common"
extension-element-prefixes="exsl">

<xsl:strip-space elements="*"/>
<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>

<xsl:variable name="Items">
    <Item ID="answer">newid_answer</Item>
    <Item ID="number">newid_number</Item>
</xsl:variable>

<xsl:key name="ItemDef-by-ID" match="ItemDef" use="@ID"/>
<xsl:key name="EnumItem-by-EID" match="EnumItem" use="@EID"/>

<xsl:variable name="xml" select="/"/>

<xsl:template match="/XML">
    <xsl:copy>
        <Items>
            <xsl:copy-of select="$Items" /> 
        </Items>
        <xsl:apply-templates select="exsl:node-set($Items)/Item"/>
    </xsl:copy>
</xsl:template>

<xsl:template match="Item">
    <xsl:variable name="item-id" select="@ID"/>

    <xsl:variable name="EnumItemRef">
        <xsl:for-each select="$xml">
            <xsl:copy-of select="key('ItemDef-by-ID', $item-id)/EnumItemRef"/>
        </xsl:for-each>
    </xsl:variable>

    <xsl:variable name="EID" select="exsl:node-set($EnumItemRef)/EnumItemRef/@EID"/>

    <ItemDef ID="{.}">
        <xsl:copy-of select="$EnumItemRef"/>
    </ItemDef>
    <xsl:for-each select="$xml">
        <xsl:copy-of select="key('EnumItem-by-EID', $EID)"/>
    </xsl:for-each>
</xsl:template>

</xsl:stylesheet>

Note: when you are in the context of a node-set created by the exslt:node-set() function, you cannot use a key() to lookup data in the source XML document, unless you switch the context to the source document first.