[xsl] A tricksy XSL question: key(name,match,use="contains()")

[David Priest <priest@xxxxxx>]

After extensive web searching, I can not find a direct answer to this  
problem (which, I rather suspect, means it is not solvable -- at  
least, not using the approach I'm attempting!).

I have an XML file in which sections are given an "@ids" attribute  
which look just like this:
  <section ids="this-is-an-unfriendly-id  this-is-the-friendly-id">

I need to identify the page on which the section starts, ie. for in- 
text references to that section.

In those cases where the section contains only a single ID, the  
following works:
  <xsl:key name="sectionkey" match="section" use="@ids" />
  <fo:page-number-citation ref-id="{key('sectionkey', @refid)/./ 
@ids}" />

However, as shown above, some sections have two ids (one from the  
actual title as written, and an abbreviated one that will continue to  
work even if someone were to change the title).  Thus, I need to  
modify the XSL so that if @ids *contains* the refid, it is a match.   
Something like -- but this doesn't work! -- the following:
  <xsl:key name="sectionkey" match="section" use="contains(@ids,  
$refid)" />
  <fo:page-number-citation ref-id="{key('sectionkey', contains(@ids,  
@refid)/./@ids" />

Is there a solution to this problem?  Either way, it should probably  
appear in the FAQ,
  http://www.dpawson.co.uk/xsl/sect2/N4852.html

Any hints and tips appreciated.

Footnote: Jeni's answer follows.  I am documenting this question and  
answer for future reference; please do add your solutions, too!

Hi David,

> However, as shown above, some sections have two ids (one from the
> actual title as written, and an abbreviated one that will continue to
> work even if someone were to change the title).  Thus, I need to
> modify the XSL so that if @ids *contains* the refid, it is a match.

As you've found, you can only use keys to locate nodes whose key value
*exactly equals* a value you're searching for: you can't use
contains() semantics.

All that means is that, in general, you need to use normal XPath
expressions to get at the relevant section, as in something like:

  <fo:page-number-citation
    ref-id="{//section[contains(@ids, $refid)]}" />

(though probably not that, since you don't want all the content of the
<section> element within the ref-id attribute).

However, what you have is a more specialised situation, where a
<section> can either have one or two ids in the ids attribute, and if
there are two then they are space-separated. That being the case, you
can construct a sectionkey key that indexes each of the sections by
the appropriate ID using string manipulation functions. For example:

<!-- sections that only have one id are indexed by that id -->
<xsl:key name="sectionkey"
         match="section[not(contains(@ids, ' '))]"
         use="@ids" />


<!-- sections that have two ids are indexed by the first and the
     second -->
<xsl:key name="sectionkey"
         match="section[contains(@ids, ' ')]"
         use="substring-before(@ids, ' ')" />
<xsl:key name="sectionkey"
         match="section[contains(@ids, ' ')]"
         use="substring-after(@ids, ' ')" />

Then you can do:

   <fo:page-number-citation ref-id="{key('sectionkey', @refid)}" />

as normal.

Cheers,

Jeni

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