Brown, William S wrote:
Esteemed experts,
There are a number of examples out there on the net of modelling
'many-to-many' relationship in XML.
A recent one is
http://en.wikibooks.org/wiki/XML:_Managing_Data_Exchange/The_many-to-many_relationship
and it is helpful, as far is it goes.
However, Kevin Williams points out in
http://www-128.ibm.com/developerworks/xml/library/x-xdm2m.html
that the usual understanding of the 'many-to-many' relationship is that it is
a two-way street.
That is, to use an example from the first document, I can structure data
about movies and actors such that each actor's info is represented only once
in the XML but pulled out repeatedly as I generate a list of movies and the
many actors that appeared in each; same actors appear in multiple movies.
Or, to use the classic example, I can represent details for each part only
once, but re-use it as I generate invoices, each of which contains many of
the same parts.
Each of the above articles suggests an ID/IDREF approach for doing this.
However, Williams notes that in relational databases it is simple to run the
query in reverse, i.e., list the movies that each actor has appeared in, or
list the invoices on which each part appears. Since the ID/IDREF is treated
as unidirectional, he suggests a double IDREF structure - paired pointers
pointing in opposite directions.
Which brings me to my question -
has anyone seen any sample xml/xsl that actually demonstrated running the
query in forward and reverse. Everything I've seen uses ID/IDREF and xsl:key
in one direction and calls that a 'many-to-many' structure. But I'm not
clever enough to navigate back upstream (William implies it can't be done) or
to implement the XSL for the dual ID/IDREF.
The first (wiki) article contains sample xml and xsl for anyone who wants to
investigate the issue.
I hope you find this diverting (or at least can show me how to do it LOL).
Best,
William S. Brown wsbrown(_at_)bnl(_dot_)gov
Systems Engineering and Safety Analysis Group
Brookhaven National Laboratory
Building 130 TEL 631 344 7230
Upton, NY 11973-5000 FAX 631 344 4900
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Hi William,
I read the many-to-many problem as two one-to-many problems, one in each
direction, for which keys are a good solution.
As far as I know, it is not possible to use a key in reverse, so two
keys must be used; one for each direction.
I include an example involving parts and invoices. Is this the kind of
thing you had in mind?
# r
----- Input data:
<doc>
<invoices>
<invoice id="i1">
<part id="1"/>
<part id="2"/>
</invoice>
<invoice id="i2">
<part id="5"/>
<part id="2"/>
</invoice>
<invoice id="i3">
<part id="5"/>
<part id="3"/>
</invoice>
<invoice id="i4">
<part id="5"/>
<part id="2"/>
</invoice>
<invoice id="i5">
<part id="3"/>
<part id="4"/>
</invoice>
</invoices>
<parts>
<part name="part1" id="1"/>
<part name="part2" id="2"/>
<part name="part3" id="3"/>
<part name="part4" id="4"/>
</parts>
</doc>
----- Stylesheet:
<?xml version="1.0" encoding="ISO-8859-1"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
version="1.0">
<xsl:output method="html" indent="no" encoding="ISO-8859-1"/>
<xsl:key name="p-i" match="/doc/invoices/invoice" use="part/@id"/>
<xsl:key name="i-p" match="/doc/invoices/invoice/part" use="../@id"/>
<xsl:template match="invoice">
<h3>invoice <xsl:value-of select="@id"/> contains:</h3>
<xsl:for-each select="key('i-p', @id)">
part <xsl:value-of select="@id"/>
</xsl:for-each>
</xsl:template>
<xsl:template match="part">
<h3>part <xsl:value-of select="@id"/> was part of invoices:</h3>
<xsl:for-each select="key('p-i', @id)">
<xsl:value-of select="@id"/>
</xsl:for-each>
</xsl:template>
<xsl:template match="doc">
<h2>invoice to parts</h2>
<xsl:apply-templates select="invoices/*"/>
<h2>part to invoices</h2>
<xsl:apply-templates select="parts/*"/>
</xsl:template>
</xsl:stylesheet>
----- End.
--
Ronan Klyne
Business Collaborator Developer
Tel: +44 (0)870 163 2555
ronan(_dot_)klyne(_at_)groupbc(_dot_)com
www.groupbc.com
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