XSLT, 2nd Edition

[2.0] The idref() Function

In the previous stylesheet, it was tedious to use the id() function in reverse, going from something with a given ID to the elements that reference it. Because this is a fairly common task, XSLT 2.0 adds the idref() function. Given an ID, idref() returns all of the elements that reference it. Here’s a simple stylesheet that works with our parts list:

<?xml version="1.0"?>
<!-- idref.xsl -->
<xsl:stylesheet version="2.0"
  xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

  <xsl:output method="text"/>

  <xsl:template match="/">
    <xsl:text>&#xA;Here is a test of the idref() </xsl:text>
    <xsl:text>function:&#xA;</xsl:text>

    <xsl:for-each select="/parts-list/part">
      <xsl:text>&#xA;  </xsl:text>
      <xsl:value-of select="name"/>
      <xsl:text> (part #</xsl:text>
      <xsl:value-of select="@part-id"/>
      <xsl:text>) is used in these products:&#xA;</xsl:text>
      <xsl:value-of select="idref(@part-id)/../../name"
        separator="&#xA;"/> 
      <xsl:text>&#xA;</xsl:text>
    </xsl:for-each>
  </xsl:template>
</xsl:stylesheet>

In this stylesheet, we can do everything with a single <xsl:value-of> element. Notice that the idref() function returns the matching attributes; that’s why we use the XPath expression idref(@part-id)/../../name to get the name of the component. The parent of the attribute is a <partref> element and its parent is a <component>. The component’s <name> child is what we want here.

Finally, the new idref() function works with the IDREFS datatype, just like id().