Schematron: Nesting XPath Values Within an XPath Predicate

In previous examples, we have seen the usage of a temporary variable or <let> tag to store a value which is later used in an XPath predicate (the square brackets surrounding the index of an element array).  It is important to note that this is not required.  A simple XPath statement can be used in the predicate for any other XPath statement.  For example see the following:

<pattern id="wEmptyMetadataComment">
  <title>Ensure person metadata comment is not blank.</title>
  <rule context="/ns:SomeDocument/nc:Person">
    <assert test="string-length(/ns:SomeDocument/nc:Metadata[@s:id=current()/@s:metadata]/nc:CommentText) &gt; 0">
      Comments regarding a person should not be blank.
    </assert>
  </rule>
</pattern>

In the above example, simply the attribute @s:id=/ns:SomeDocument/nc:Person/@s:metadata is used to identify which specific Metadata element should be examined.  With the context defined as /ns:SomeDocument/nc:Person, the rule will loop through each nc:Person element and use the appropriate @s:metadata value in each subsequent pass.

[Updated: Corrected Syntax on 04-01-2010]