need to merge data from two xml files based on record id - xslt-3.0

Question

i have the following two xml files.
File A:
<breakfast_menu>
<food>
<id>1</id>
<name>Belgian Waffles</name>
<price>$5.95</price>
<calories>650</calories>
</food>
<food>
<id>2</id>
<name>Strawberry Belgian Waffles</name>
<price>$7.95</price>
<calories>900</calories>
</food>
</breakfast_menu>
and file B:
<breakfast_menu>
<food>
<id>1</id>
<description>here we have already a description element</description>
<otherData>s3650</otherData>
</food>
<food>
<id>2</id>
<otherData>s3250</otherData>
<otherData2>g508</otherData2>
<otherData3>sidi6098</otherData3>
</food>
</breakfast_menu>
what i cannot achieve, is for each record that matches based on the record id, take all info from the xml file named A, and put all this info in the description element of file B, but also keep the element it came from. If the matching record does not have a description element, we add one. If it already has one, then we must be aware of where the info taken from file A starts, with a textual separator, ie "-followingInfoTakenFromFileA-". So with the above in mind, the desired output should be like:
<breakfast_menu>
<food>
<id>1</id>
<description>here we have already a description element-followingInfoTakenFromFileA-name:Belgian Waffles-price:$5.95-calories:650</description>
<otherData>s3650</otherData>
</food>
<food>
<id>2</id>
<otherData>s3250</otherData>
<otherData2>g508</otherData2>
<otherData3>sidi6098</otherData3>
<description>name:Strawberry Belgian Waffles-price:$7.95-calories:900</description>
</food>
</breakfast_menu>

Answer 1

It seems, given the sorted id merge keys, a simple task for xsl:merge:
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
exclude-result-prefixes="#all"
expand-text="yes"
version="3.0">
<xsl:param name="doc-A">
<breakfast_menu>
<food>
<id>1</id>
<name>Belgian Waffles</name>
<price>$5.95</price>
<calories>650</calories>
</food>
<food>
<id>2</id>
<name>Strawberry Belgian Waffles</name>
<price>$7.95</price>
<calories>900</calories>
</food>
</breakfast_menu>
</xsl:param>
<xsl:mode on-no-match="shallow-copy"/>
<xsl:output method="xml" indent="yes"/>
<xsl:template match="/*">
<xsl:copy>
<xsl:merge>
<xsl:merge-source for-each-item="." select="food">
<xsl:merge-key select="id"/>
</xsl:merge-source>
<xsl:merge-source for-each-item="$doc-A" select="//food">
<xsl:merge-key select="id"/>
</xsl:merge-source>
<xsl:merge-action>
<xsl:copy>
<xsl:apply-templates select="*">
<xsl:with-param name="merge-data"
select="let $data := string-join(current-merge-group()[2]/(* except id)!(name() || ':' || .), '-')
return if (description)
then '-followingInfoTakenFrom' || document-uri(root(current-merge-group()[2])) || $data
else $data"/>
</xsl:apply-templates>
</xsl:copy>
</xsl:merge-action>
</xsl:merge>
</xsl:copy>
</xsl:template>
<xsl:template match="food/description">
<xsl:param name="merge-data"/>
<xsl:copy>{.}{$merge-data}</xsl:copy>
</xsl:template>
<xsl:template match="food[not(description)]/*[last()]">
<xsl:param name="merge-data"/>
<xsl:next-match/>
<description>{$merge-data}</description>
</xsl:template>
</xsl:stylesheet>
https://xsltfiddle.liberty-development.net/pPzifqo
That examples inlines the first document as a parameter for self-containedness of the sample but in the real world you could load from a URL with e.g. <xsl:param name="doc-A" select="doc('fileA.xml')"/>.