Sorting on attribute collected from reference list - xslt

Question

I am trying to sort a list of categorized xml elements, using XSLT 2.0. Each element has a unique ID and the categorization is defined in another list containing these and more elements. Here's an example of a starting XML document. The section that I want sorted is /Atlas/VisitedCities. It should be sorted according to area of the world and date of the visit:
<?xml version="1.0" encoding="UTF-8"?>
<Atlas>
<Cities>
<City id="1" worldPart="Africa">
<Name>Luxor</Name>
<Founded>-3200</Founded>
<Location>Egypt</Location>
</City>
<City id="2" worldPart="Africa">
<Name>Tripoli</Name>
<Founded>-700</Founded>
<Location>Libya</Location>
</City>
<City id="3" worldPart="Americas">
<Name>Cholula</Name>
<Founded>-200</Founded>
<Location>Mexico</Location>
</City>
<City id="4" worldPart="Americas">
<Name>Flores</Name>
<Founded>-1000</Founded>
<Location>Guatemala</Location>
</City>
<City id="5" worldPart="Europe">
<Name>Argos</Name>
<Founded>-5000</Founded>
<Location>Greece</Location>
</City>
<City id="6" worldPart="Europe">
<Name>Athens</Name>
<Founded>-4000</Founded>
<Location>Greece</Location>
</City>
</Cities>
<VisitedCities lastUpdate="2018-09-10">
<VisitedCity cityID="6">
<Date>1883-08-26</Date>
<Visitor>Dora</Visitor>
</VisitedCity>
<VisitedCity cityID="3">
<Date>1907-01-02</Date>
<Visitor>Nemo</Visitor>
</VisitedCity>
<VisitedCity cityID="4">
<Date>1940-02-08</Date>
<Visitor>Jimenez</Visitor>
</VisitedCity>
<VisitedCity cityID="2">
<Date>1886-06-10</Date>
<Visitor>James T. Kirk</Visitor>
</VisitedCity>
</VisitedCities>
</Atlas>
The wanted output is this:
<?xml version="1.0" encoding="UTF-8"?>
<Atlas>
<Cities>
<City id="1" worldPart="Africa">
<Name>Luxor</Name>
<Founded>-3200</Founded>
<Location>Egypt</Location>
</City>
<City id="2" worldPart="Africa">
<Name>Tripoli</Name>
<Founded>-700</Founded>
<Location>Libya</Location>
</City>
<City id="3" worldPart="Americas">
<Name>Cholula</Name>
<Founded>-200</Founded>
<Location>Mexico</Location>
</City>
<City id="4" worldPart="Americas">
<Name>Flores</Name>
<Founded>-1000</Founded>
<Location>Guatemala</Location>
</City>
<City id="5" worldPart="Europe">
<Name>Argos</Name>
<Founded>-5000</Founded>
<Location>Greece</Location>
</City>
<City id="6" worldPart="Europe">
<Name>Athens</Name>
<Founded>-4000</Founded>
<Location>Greece</Location>
</City>
</Cities>
<VisitedCities lastUpdate="2018-09-10">
<VisitedCity cityID="2">
<Date>1886-06-10</Date>
<Visitor>James T. Kirk</Visitor>
</VisitedCity>
<VisitedCity cityID="6">
<Date>1883-08-26</Date>
<Visitor>Dora</Visitor>
</VisitedCity>
<VisitedCity cityID="3">
<Date>1907-01-02</Date>
<Visitor>Nemo</Visitor>
</VisitedCity>
<VisitedCity cityID="4">
<Date>1940-02-08</Date>
<Visitor>Jimenez</Visitor>
</VisitedCity>
</VisitedCities>
</Atlas>
The stylesheet (XSLT 2.0) that I am struggling with looks like this:
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
version="2.0">
<!-- Format output -->
<xsl:output method="xml" indent="yes"/>
<xsl:strip-space elements="*" />
<!-- Copy everything that does not match later templates. -->
<xsl:template match="node()|#*" priority="-1">
<xsl:copy>
<xsl:apply-templates select="node()|#*"/>
</xsl:copy>
</xsl:template>
<xsl:variable name="city.list" select="/Atlas/Cities"/>
<xsl:variable name="sort-order" as="element()*">
<wPart>Africa</wPart>
<wPart>Europe</wPart>
<wPart>Americas</wPart>
</xsl:variable>
<xsl:template match="/Atlas/VisitedCities">
<xsl:variable name="city-list" select="."/>
<xsl:copy>
<xsl:apply-templates select="#*"/>
<xsl:for-each select="$sort-order">
<xsl:variable name="this-wpart" select="./text()"/>
<!-- How to select VisitedCity based on info in other list??? -->
<xsl:apply-templates select="$city-list/VisitedCity[$city.list/City[#cityID=$city-list/VisitedCity/#cityID]/#worldPart=$this-wpart]">
<xsl:sort select="./Date"/>
</xsl:apply-templates>
</xsl:for-each>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>
I think I understand why this stylesheet will not work (it does not sort at all), as I don't know how to make the selection in the (last) apply-templates. I don't see how to refer to the outermost elements from the inner parts of this expression.

Answer 1

It might suffice to set up a key to reference the City elements by the id attribute to then, in the xsl:sort select expression reference the worldPart attribute. Additionally you could replace the for-each on your continent order with an index-of() call with
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
exclude-result-prefixes="#all"
version="3.0">
<xsl:mode on-no-match="shallow-copy"/>
<xsl:output method="xml" indent="yes"/>
<xsl:key name="city-by-id" match="Cities/City" use="#id"/>
<xsl:variable name="sort-order" as="element()*">
<wPart>Africa</wPart>
<wPart>Europe</wPart>
<wPart>Americas</wPart>
</xsl:variable>
<xsl:template match="VisitedCities">
<xsl:copy>
<xsl:apply-templates select="VisitedCity">
<xsl:sort select="index-of($sort-order, key('city-by-id', #cityID)/#worldPart)"/>
<xsl:sort select="Date"/>
</xsl:apply-templates>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>
https://xsltfiddle.liberty-development.net/eiZQaFJ
That complete example is XSLT 3 but to use it with XSLT 2 you would just replace the xsl:mode declaration in there with your template you have prefixed with the comment <!-- Copy everything that does not match later templates. -->, that is, with the identity transformation template.