Re: [xsl] key with restricting the scope of use attribute

[Jeni Tennison <mail at jenitennison dot com>]

Hi Dan,

Let's put it this way...

When you have a key that uses the generated ID of the node that it
matches, the key value can only ever get that single node - none of
the rest of the use expression matters, aside from as a test on the
values that you use in them.

In more detail, when you do:

<xsl:key name="MyKey1" match="a/b"
         use="concat(generate-id(),':',@x,':',@y)"/>

<xsl:template match="a/b">
   <xsl:for-each select="key('MyKey1',concat(generate-id(),':','1:1'))">
      MyKey1:@id=<xsl:value-of select="@id"/><br/>
   </xsl:for-each>
   ...
</xsl:template>

In the template you're looking at a particular b element.  This b
element will be indexed in the key according to its generated ID, its
x attribute and its y attribute.  So given the b element:

  <b id="1" x="1" y="1" />

the key will be something like:

  randomID1:1:1

*Only* this particular b element will have this key because
generate-id() produces a unique ID for a node.  Thus each key value
will only ever access one node.
  
When you select nodes using the key, the evaluation of the
generate-id() for the b element is going to be exactly the same as it
was for the key.  So for the above b element, the call looks like:

  key('MyKey1', 'randomID1:1:1')

The only node this could possibly retrieve is the one identified by
that particular unique ID (i.e. the b element above).  It won't
retrieve anything, though, if the x attribute and y attributes are not
both equal to 1, as it cannot match any nodes.

So basically, you're either retrieving the node itself or nothing at
all - it comes down to a test of whether the node's x and y attributes
are both 1 or not.  And that means that it's exactly equivalent to
doing:

<xsl:template match="a/b">
   <xsl:if test="@x = 1 and @y = 1">
      MyKey1:@id=<xsl:value-of select="@id" /><br />
   </xsl:if>
</xsl:template>

Usually you use generate-id() in a use expression because you want to
restrict the nodes that you retrieve from a key to a subtree of the
document - you use the unique ID of the parent of the nodes that
you're indexing, or one of its ancestors, or feasibly a descendant -
something where *lots* of nodes could have the same node related to
them.

[BTW, if you've got id attributes, it's more efficient to use them
rather than generating unique IDs all the time.]

I hope that helps explain why we're struggling to understand what
you're aiming for with this particular question.

Cheers,

Jeni

---
Jeni Tennison
http://www.jenitennison.com/



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