You're trying to do a join. That involves comparing items from two different
sets.
In SQL (and in mathematical predicate logic) you do this by binding "range
variables" to the items in each set, and then using predicates such as
where X.ref = Y.id
You can use the same approach in XSLT, but with caveats. Only with XSLT 3.0 are
you allowed to bind variables within an XPath expression. But that's where "."
and "current() come in. Within a predicate, "." is used as an implicit range
variable for the set you are filtering. You can also sometimes use "current()"
as your second range variable. Or you can bind variables explicitly using
xsl:variable. The problem about using ".", however, is that it changes its
meaning in a nested predicate.
In this case in XSLT 2.0 upwards, current() rescues you:
match="type[.='Action'][$data/device-description/param[@name=current()/../base-name]/@standard='no')]
"
(Note I've avoided the use of text() here. It's much better to compare with the
string-value of the element, rather than with its text nodes - it will continue
to work, for example, if the text is split up by comments).
XSLT 2.0 defines that current() within a pattern refers to the node being
tested against the pattern. Unfortunately though (IIRC), XSLT 1.0 didn't allow
use of current() in patterns. But no-one is still using XSLT 1.0, are they???
Michael Kay
Saxonica
On 17 Jan 2018, at 15:19, Kerry, Richard
richard(_dot_)kerry(_at_)atos(_dot_)net
<mailto:richard(_dot_)kerry(_at_)atos(_dot_)net>
<xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com
<mailto:xsl-list-service(_at_)lists(_dot_)mulberrytech(_dot_)com>> wrote:
I am trying to use XSLT to process one XML file, which refers to data in
another, and I am having trouble with getting a filter to work.
The first source file (device.xml) - the one that is actually processed by
the XSL - has entries like this:
<device-description>
<device-value>
<base-name>PollNow</base-name>
<name>Poll Now</name>
<type>Action</type>
</device-value>
<device-value>
<base-name>StartupPollNow</base-name>
<name>Startup Poll Now</name>
<type>Action</type>
</device-value>
<device-value>
<base-name>Ping</base-name>
<name>Ping</name>
<type minimum-length="0" maximum-length="256" >String</type>
</device-value>
</device-description>
One of the templates is approximately as follows:
<x:template match="type[text()='Action']" >
<!-- do basic action. -->
</x:template>
This quite happily matches all the <device_name> elements where the <type>
has a text value pf "Action".
However, I have a second XML file (device-3.xml) which contains some data
that I wish to use to modify the behaviour of the transform. It has entries
like this:
<device-description>
<param name="PollNow" standard="no" >
<name>Poll Now</name>
<description>normal poll</description>
</param>
<param name="StartupPollNow" standard="no" >
<name>Startup Poll Now</name>
<description>startup poll</description>
</param>
<param name="Ping">
<name>Ping</name>
<description>Ping-Pong</description>
</param>
</device-description>
The second file is accessed via a variable, as follows:
<x:variable name="data" select="document($in-file-3)"/>
What I want to achieve is that for all <type> elements where there is a
corresponding <param> where base-name is the same as param/@name, I want a
different action.
The additional template I have for this is currently as follows
<x:template
match="type[text()='Action'][$data/device-description/param[@name=../base-name/text()]/@standard='no')]
" >
<!-- do different action. -->
</x:template>
My thinking is that the first filter is as with the first template I listed,
matching all <type> elements whose text is "Action".
The second filter should then enable me to pick the right param from the
second file, and do something based on the value of its @standard.
Maybe once I'm in the second filter I am causing confusion as to which
element the @name refers to (which should be the param) and which the
../base-name is relative to (the device-value/type)?
Can I do all this within a single template's @match ? Or am I making it too
complicated. In which case how might I get the result I'm looking for?
Appreciatively,
Richard.
The examples are all slightly simplified from the real data, which has more
elements, and the transforms are in a mode.
Richard Kerry
BNCS Engineer, SI SOL Telco & Media Vertical Practice
T: +44 (0)20 3618 2669
M: +44 (0)7812 325518
4 Triton Square, Regent’s Place, London NW1 3HG
richard(_dot_)kerry(_at_)atos(_dot_)net
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