I have been attempting to group child nodes using the Munechian method but am now stumped.

I am trying to convert this XML
<PublisherList>
  <publisher>
    <name>Penguin</name>
    <books>
      <book>
        <title>Title One</title>
        <author>David</author>
      </book>
      <book>
        <title>Title Two</title>
        <author>John</author>
      </book>
      <book>
        <title>Title Three</title>
        <author>John</author>
      </book>
      <book>
        <title>Title Four</title>
        <author>David</author>
      </book>
    </books>
  </publisher>
  <publisher>
    <name>Wrox</name>
    <books>
      <book>
        <title>Title Five</title>
        <author>David</author>
      </book>
      <book>
        <title>Title Six</title>
        <author>John</author>
      </book>
      <book>
        <title>Title Seven</title>
        <author>John</author>
      </book>
      <book>
        <title>Title Eight</title>
        <author>David</author>
      </book>
    </books>
  </publisher>
</PublisherList>

To this:
<List>
    <BookPublisher>
        <name>Penguin</name>
        <BooksByAuthor>
            <AuthorName>David</AuthorName>
            <title>Title One</title>
            <title>Title Four</title>
        </BooksByAuthor>
        <BooksByAuthor>
            <AuthorName>John</AuthorName>
            <title>Title Two</title>
            <title>Title Three</title>
        </BooksByAuthor>
    </BookPublisher>
    <BookPublisher>
        <name>Wrox</name>
        <BooksByAuthor>
            <AuthorName>David</AuthorName>
            <title>Title Five</title>
            <title>Title Eight</title>
        </BooksByAuthor>
        <BooksByAuthor>
            <AuthorName>John</AuthorName>
            <title>Title Six</title>
            <title>Title Seven</title>
        </BooksByAuthor>
    </BookPublisher>
</List>

using this XSLT (so far)
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
    <xsl:output method="xml"/>

    <xsl:key name="publishers" match="publisher" use="name" />
    <xsl:key name="authors" match="author" use="." />

    <xsl:template match="/">
        <xsl:element name="List">

            <xsl:for-each select="//publisher[count(. | key('publishers', name)[1]) = 1]">
                <BookPublisher>
                    <xsl:variable name="publisher" select="name"/>
                    <name><xsl:value-of select="$publisher"/></name>

                <xsl:for-each select=".//author[count(.| key('authors',.)[1])=1]">
                    <AuthorName><xsl:value-of select="."/></AuthorName>
                </xsl:for-each>
                </BookPublisher>
            </xsl:for-each>

        </xsl:element>

    </xsl:template>

</xsl:stylesheet>


But I get this result:
<?xml version="1.0"?>
<List>
    <BookPublisher>
        <name>Penguin</name>
        <AuthorName>David</AuthorName>
        <AuthorName>John</AuthorName>
    </BookPublisher>
    <BookPublisher>
        <name>Wrox</name>
    </BookPublisher>
</List>

Can anyone explain why on the second pass through the for-each statment, the AuthorNames are not being extracted that are unique for Wrox. The only way I can make it work is by changing the authors key --> <xsl:key name="authors" match="publisher[name='Wrox']//author" use="." />

But I can't use a variable (name = $name)

Any help or insight would be appreciated

I'm not sure why you're looking for distinct publishers, you don't have any duplicates in your data, and I'm not sure how you would want to handle them if there were any.

Using Muenchian grouping within a subtree, rather than within the full document, is always a little bit tricky. The usual answer is to define a key that makes the name globally unique, in this case concatenating the publisher name and the author name.

Where it's going wrong is here

<xsl:for-each select=".//author[count(.| key('authors',.)[1])=1]">

where you look for authors within the current publisher, and then test them to see if they are the first author with that name in the document. Try changing the key on author use="concat(ancestor::publisher/name, '#', .)", and changing the above to

select=".//author[count(.| key('authors',concat(current()/name, '#', .)[1])=1]">

I don't guarantee that's enough to make it work but it's the essence of the solution.

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference

Thats great! Thanks. I can't quite get it to work (its telling me that the expression, select=".//author[count(.| key('authors', concat(current()/name, '#', .)[1]=1)]" must evaluate to a node-set). I'm using MSXML 4.0 and can't seem to fit the node-set extension in the right placem but I think I can find my way now

Thanks so much for the insight!

I got it working! Thanks. I needed to change current() to a variable, but now its perfect. Thanks again

final version that works (now I can add the rest)

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">

    <xsl:output method="xml"/>
    <xsl:key name="authors" match="author" use="concat(ancestor::publisher/name, '#', .)" />
    <xsl:template match="/">
        <xsl:element name="List">
            <xsl:for-each select="//publisher">
                <xsl:variable name="publisher" select="name"/>
        <p><xsl:value-of select="$publisher"/></p>
                <xsl:for-each select=".//author[count(.| key('authors', concat($publisher, '#', .))[1])=1]">
                 <b><xsl:value-of select="."/></b>
                </xsl:for-each>
            </xsl:for-each>
        </xsl:element>
    </xsl:template>
</xsl:stylesheet>