How to Specify XPath for XML unique Constraint

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XML

Hi,

I am unable to get the xs:unique specifier to work in an XML file. I just don't seem to be able to work out an XPath that works. My apologies for the amount of code in this question, but I would be extremely grateful to anyone who can point out what I am doing wrong below. No matter what I do, I cannot get the @ref attribute in the <artwork> element to report an error for my duplicating the value (each ref must be unique).

As an aside, does anyone know of a good XPath evaluation tool? This seems like it would be such a useful thing to have that I can't believe someone hasn't knocked something together.

Any help or pointers to information would be very gratefully received.

Kind wishes, Patrick

This is my schema:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema id="Artworks"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
    xmlns:aw="http://www.fourthwish.co.uk/data/Artworks.xsd"
    xmlns="http://www.fourthwish.co.uk/data/Artworks.xsd"
    targetNamespace="http://www.fourthwish.co.uk/data/Artworks.xsd"
    elementFormDefault="qualified"
>
    <xs:element name="artworks">
        <xs:complexType>
            <xs:sequence minOccurs="0" maxOccurs="unbounded">
                <xs:element name="artwork" type="ArtworkType">
                    <xs:unique name="uniqueRef">
                        <xs:selector xpath="."/>
                        <xs:field xpath="@ref"/>
                    </xs:unique>
                </xs:element>
            </xs:sequence>
        </xs:complexType>
    </xs:element>
    <xs:complexType name="ArtworkType">
        <xs:sequence>
            <xs:element name="title" type="xs:string"/>
        </xs:sequence>
        <xs:attribute name="ref" type="xs:nonNegativeInteger"/>
    </xs:complexType>
</xs:schema>

And this is my XML file:

<?xml version="1.0" encoding="utf-8" standalone="yes"?>
<artworks
    xmlns="http://www.fourthwish.co.uk/data/Artworks.xsd"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://www.fourthwish.co.uk/data/Artworks.xsd Artworks.xsd"
>
    <artwork ref="1">
        <title>Title String</title>
    </artwork>
    <artwork ref="1">
        <title>Title String</title>
    </artwork>
</artworks

>