XML Schema Identity constraints - Can I use absolute path in XPath in selector?

Question

I want to implement referential integrity in a xml document using key & keyref in a XML Schema.

Let's say my XML looks like this:

<root>
  <parents>
     <parent parentID="P01">XXX</parent>
     <parent parentID="P02">XXX</parent>
  </parents>

  <sons>
     <son sonID="S01" parentID="P01">XXX</son>
     <son sonID="S02" parentID="P02">XXX</son>
     <son sonID="S03" parentID="P02">XXX</son>
  </sons>
</root>

In my XML, for example, I want to define the parentID attribute as a primary key for the parent element.

I declare the corresponding xs:key inside the root element:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">

<xs:element name="root" type="myRootType" >
  <xs:key name="parentKey">
    <xs:selector xpath="./parents/parent" />
    <xs:field xpath="@parentID"/>
  </xs:key>  
</xs:element>

<xs:complexType name="myRootType">
   <xs:sequence>
   ....

It works fine (validation gives an error if two parents have the same ID) using a relative path ("./parents/parent) in the xpath attribute of the xs:selector element, as in the example shown above.

But the identity restriction doesn't work if I use the absolute path instead ("/root/parents/parent").

Why is this? my understanding whas that it should work in both ways (using abosolute or relative paths in the XPath expression.

Answer 1

Actually XML Schema doesn't support any XPath expression in <xs:selector>, but a subset which doesn't include expressions that start with /. That is formally described here.

Besides not working, your processor should have produced an error message since /root/parents/parent is an invalid expression.