About using xs:unique on xsi:type in a sequence

Question

It is possible to have a unique constraint on an element\'s type ?

Let\'s say I have a Noah\'s Ark where Animal/@name must be unique.

Below\'s an XML that do not

Answer 1

All the implementations I could test on XSD 1.0 seem to agree on one thing here: the identity constraints will be tested only if the selectors/fields match XML nodes for which an associated user defined schema component can be found. xsi:type, while it is a schema related markup, is not considered here since its intention is different. It might prove academically interesting to check if a Schematron assertion might help here...

UPDATE: I am including the Schematron version, too; I am running the ISO version, XSLT1 w/ Microsoft extensions since I am on .NET.

<?xml version="1.0"?>
<sch:schema xmlns:sch="http://purl.oclc.org/dsdl/schematron" xmlns:ms="urn:schemas-microsoft-com:xslt">
    <sch:ns uri="http://www.w3.org/2001/XMLSchema-instance" prefix="xsi"/>
    <sch:ns uri="urn:schemas-microsoft-com:xslt" prefix="ms"/>

   <sch:pattern id="about-using-xsunique-on-xsitype-in-a-sequence">
      <sch:rule context="//Animal/@xsi:type">
        <sch:let name="targetNodeSet" value="//Animal/@xsi:type"/>
        <sch:assert test="count($targetNodeSet[concat('{', ms:namespace-uri(.), '}', ms:local-name(.)) = concat('{', ms:namespace-uri(current()), '}', ms:local-name(current()))]) = 1">
            Only one-of-a-kind animal is allowed.</sch:assert>
        <sch:assert test="count($targetNodeSet[. = current()]) = 1">
            Only one-of-a-kind animal is allowed (naive).</sch:assert>
      </sch:rule>
   </sch:pattern>
</sch:schema>

I've defined two assertion, a "naive" one that textually compares the value of the xsi:type attribute; and the "correct" one which compares them as QName s.

For this XML:

<ns:NoahsArk xmlns:ns1="http://www.xxx.com" xmlns:ns="http://www.xxx.com" xmlns:xs="http://www.w3.org/2001/XMLSchema-instance" xs:schemaLocation="http://www.xxx.com Persons.xsd">  
    <Animal xs:type="ns:Dog" ns:name="A" ns:pedigree="caniche"/> 
    <Animal xs:type="ns1:Dog" ns:name="B" ns:pedigree="caniche"/> 
    <Animal xs:type="ns1:Dog" ns:name="C" ns:pedigree="caniche"/> 
</ns:NoahsArk> 

This is the result:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<svrl:schematron-output title="" schemaVersion="" xmlns:svrl="http://purl.oclc.org/dsdl/svrl" xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:schold="http://www.ascc.net/xml/schematron" xmlns:sch="http://www.ascc.net/xml/schematron" xmlns:iso="http://purl.oclc.org/dsdl/schematron" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:ms="urn:schemas-microsoft-com:xslt">
    <svrl:ns-prefix-in-attribute-values uri="http://www.w3.org/2001/XMLSchema-instance" prefix="xsi"/>
    <svrl:ns-prefix-in-attribute-values uri="urn:schemas-microsoft-com:xslt" prefix="ms"/>
    <svrl:active-pattern id="about-using-xsunique-on-xsitype-in-a-sequence" name="about-using-xsunique-on-xsitype-in-a-sequence"/>
    <svrl:fired-rule context="//Animal/@xsi:type"/>
    <svrl:failed-assert test="count($targetNodeSet[concat(', ms:namespace-uri(.), ', ms:local-name(.)) = concat(', ms:namespace-uri(current()), ', ms:local-name(current()))]) = 1" location="/@*[local-name()='type' and namespace-uri()='http://www.w3.org/2001/XMLSchema-instance']">
        <svrl:text>
            Only one-of-a-kind animal is allowed.</svrl:text>
    </svrl:failed-assert>
    <svrl:fired-rule context="//Animal/@xsi:type"/>
    <svrl:failed-assert test="count($targetNodeSet[concat(', ms:namespace-uri(.), ', ms:local-name(.)) = concat(', ms:namespace-uri(current()), ', ms:local-name(current()))]) = 1" location="/@*[local-name()='type' and namespace-uri()='http://www.w3.org/2001/XMLSchema-instance']">
        <svrl:text>
            Only one-of-a-kind animal is allowed.</svrl:text>
    </svrl:failed-assert>
    <svrl:failed-assert test="count($targetNodeSet[. = current()]) = 1" location="/@*[local-name()='type' and namespace-uri()='http://www.w3.org/2001/XMLSchema-instance']">
        <svrl:text>
            Only one-of-a-kind animal is allowed (naive).</svrl:text>
    </svrl:failed-assert>
    <svrl:fired-rule context="//Animal/@xsi:type"/>
    <svrl:failed-assert test="count($targetNodeSet[concat(', ms:namespace-uri(.), ', ms:local-name(.)) = concat(', ms:namespace-uri(current()), ', ms:local-name(current()))]) = 1" location="/@*[local-name()='type' and namespace-uri()='http://www.w3.org/2001/XMLSchema-instance']">
        <svrl:text>
            Only one-of-a-kind animal is allowed.</svrl:text>
    </svrl:failed-assert>
    <svrl:failed-assert test="count($targetNodeSet[. = current()]) = 1" location="/@*[local-name()='type' and namespace-uri()='http://www.w3.org/2001/XMLSchema-instance']">
        <svrl:text>
            Only one-of-a-kind animal is allowed (naive).</svrl:text>
    </svrl:failed-assert>
</svrl:schematron-output>

As you can see from the failed assertions, the "naive" way is flagging only 2 of 3 instances; again, comparing the value of the xsi:type attribute as text is not the same as comparing QName s (which is what @xsi:types are).

Answer 2

Thanks for your response.

Just for fun I made an attempt with Schematron ; however I couldn't make it work since the XPath expression I find out uses "current()" which is not available in oxygen or XMLSpy, so I can't test it :(

    <?xml version="1.0" encoding="UTF-8"?>
    <schema xmlns="http://www.ascc.net/xml/schematron" xmlns:ns="http://www.xxx.com" >
        <ns prefix="ns" uri="http://www.xxx.com"/>
        <ns prefix="xs" uri="http://www.w3.org/2001/XMLSchema-instance"/>
            <pattern name="AnimalNameUnicity">
                <rule context="Animals/Animal">
                    <assert test="count(//@ns:name[ . = current()]) > 1">name not unique !</assert>        
                </rule>
            </pattern> 
    </schema>