XSLT 2.0 - Distinct values using a key

Question

I'm having a bit of troubles working on a XSLT sheet. This is my XML document :

<?xml version="1.0" encoding="UTF-8"?>
<catalog>
  <products>
    <product>
      <id>1</id>
    </product>
  </products>
  <stocks>
    <stock>
      <id>1</id>
      <size>S</size>
      <store>NYC</store>
    </stock>
    <stock>
      <id>1</id>
      <size>L</size>
      <store>NYC</store>
    </stock>
    <stock>
      <id>1</id>
      <size>S</size>
      <store>LA</store>
    </stock>
  </stocks>
</catalog>

What I want is to have this kind of output XML :

<?xml version="1.0" encoding="UTF-8"?>
<catalog>
  <products>
    <product>
      <id>1</id>
      <variants>
        <variant>
          <size>S</size>
          <stocks>
            <stock store-ref="NYC">
            <stock store-ref="LA">
          </stocks>
        <variant>
        <variant>
          <size>L</size>
          <stocks>
            <stock store-ref="NYC">
          </stocks>
        <variant>
      </variants>
    </product>
  </products>
</catalog> 

Today, I'm using this XSLT to perform this transformation :

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="2.0" exclude-result-prefixes="xs fn" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:fn="http://www.w3.org/2005/xpath-functions">
  <xsl:key name="sizes" match="stock" use="id"/>
  <xsl:key name="stocks" match="stock" use="fn:concat(id, '-', size)"/>
  <xsl:output method="xml" encoding="UTF-8" indent="yes"/>
  <xsl:template match="/">
    <catalog>
      <products>
        <xsl:for-each select="/catalog/products/product">
          <product>
            <id><xsl:value-of select="id" /></id>
            <variants>
              <xsl:for-each select="key('sizes', id)">
                <variant>
                  <size><xsl:value-of select="size" /></size>
                  <stocks>
                    <xsl:for-each select="key('stocks', fn:concat(id, '-', size))">
                      <stock store-ref="{store}" />
                    </xsl:for-each>
                  </stocks>
                </variant>
              </xsl:for-each>
            </variants>
          </product>
        </xsl:for-each>
      </products>
    </catalog>
  </xsl:template>
</xsl:stylesheet>

and I got this result :

<?xml version="1.0" encoding="UTF-8"?>
<catalog>
   <products>
      <product>
         <id>1</id>
         <variants>
            <variant>
               <size>S</size>
               <stocks>
                  <stock store-ref="NYC"/>
                  <stock store-ref="LA"/>
               </stocks>
            </variant>
            <variant>
               <size>L</size>
               <stocks>
                  <stock store-ref="NYC"/>
               </stocks>
            </variant>
            <variant>
               <size>S</size>
               <stocks>
                  <stock store-ref="NYC"/>
                  <stock store-ref="LA"/>
               </stocks>
            </variant>
         </variants>
      </product>
   </products>
</catalog>

So my problem is that I'd like to select distinct size values but it does not seem to work. I've tried to use generate-id() but I don't really understand how it works so I didn't had great results with it :( Any idea how to fix this ? Thanks !

Answer 1

Additionally to the solution you found yourself, I'd like to show you multi-level grouping.

You only would need a single key, for convenience:

<xsl:key name="kStock" match="stock" use="id"/>

and this:

<xsl:template match="product">
  <xsl:copy>
    <xsl:copy-of select="id" />
    <variants>
      <xsl:for-each-group select="key('kStock', id)" group-by="size">
        <variant>
          <xsl:copy-of select="size" />
          <stocks>
            <xsl:for-each-group select="current-group()" group-by="store">
              <stock store-ref="{current-grouping-key()}" />
            </xsl:for-each-group>
          </stocks>
        </variant>
      </xsl:for-each-group>
    </variants>
  </xsl:copy>
</xsl:template>

Note the use of current-group() and current-grouping-key().

Answer 2

Thanks to @Tomalak, I've found the solution. Here is my working XSLT :

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="2.0" exclude-result-prefixes="xs fn" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:fn="http://www.w3.org/2005/xpath-functions">
  <xsl:key name="sizes" match="stock" use="id"/>
  <xsl:key name="stocks" match="stock" use="fn:concat(id, '-', size)"/>
  <xsl:output method="xml" encoding="UTF-8" indent="yes"/>
  <xsl:template match="/">
    <catalog>
      <products>
        <xsl:for-each select="/catalog/products/product">
          <product>
            <id><xsl:value-of select="id" /></id>
            <variants>
              <xsl:for-each-group select="key('sizes', id)" group-by="size">
                <variant>
                  <size><xsl:value-of select="size" /></size>
                  <stocks>
                    <xsl:for-each select="key('stocks', fn:concat(id, '-', size))">
                      <stock store-ref="{store}" />
                    </xsl:for-each>
                  </stocks>
                </variant>
              </xsl:for-each-group>
            </variants>
          </product>
        </xsl:for-each>
      </products>
    </catalog>
  </xsl:template>
</xsl:stylesheet>