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Use care in creating Schematron rules that apply to elements withfixed or default attributes

Hi Folks,

Consider the following XML document. There are two <Attitude> elements; one has a "units" attribute:

    <Altitude units="feet" />
    <Altitude /> 

I want a Schematron rule that expresses this:

      The <Test> element must have only
      one child <Altitude> element with a
      units="feet" attribute.

So you write this Schematron rule:

<sch:rule context="Test"> 
    <sch:assert test="not(Altitude[@units eq 'feet'][2])"> 
        There is exactly one Altitude child
        element with a units="feet" attribute. 

The rule fires, the assertion succeeds, and all is fine.

Pretty simple, right?

Not so fast.

If the XML Schema declares the units attribute to be either fixed or with a default:

<xsd:attribute name="units" type="xsd:string" default="feet" />


<xsd:attribute name="units" type="xsd:string" fixed="feet" />

then there are actually two Attitude elements with a units="feet" attribute and the assertion should fail.


The Schematron rule must be revised: it must check that there is one Altitude element that has a units="feet" attribute or has no units attribute.

Lesson Learned: Schematron rules must not be created in isolation, independent of the grammar schema (DTD, XML Schema, Relax NG).


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