XML schema uniqueness constraints

[Olaf Meyer]

I'm wondering if I can also express uniquness constraints on attributes
of objects that I have referenced from somewhere else.

To be a bit more precise here a short and simple XML document. The
"objects" have a key constraint on the id attribute, i.e. they need to
be unique. The object-ref element may reference the previously defined
objects and hence has a keyref constraint on the id attribute.

I would now like to furhter restrict the object-ref elements by only
allowing refrerences, such that the class attributes of the referred
objects are unique. Is there an elegant way to do this via a XML
uniquness constraint?

<?xml version="1.0" encoding="UTF-8"?>
<root xmlns="http://www.mynames.org"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.mynames.org
x:/work/sjrodc/xml/constraint.xsd">
<my-objects>
<object id="1" class="Car"/>
<object id="2" class="Car"/>
<object id="3" class="Bike"/>
</my-objects>
<object-ref id="1"/>
<!-- How can I specify a constraint in the schema such that only
objects with distinct classes can be referenced?
i.e. only one of the objects with id 1 or 2 can be
selected -->
<!-- this should cause a constraint violation -->
<object-ref id="2"/>

<object-ref id="3"/>
</root>

I already have a schema file that has a key constraint on the objects id
attribute and a keyref constraint on the object-ref id attribute. Is it
possible to add another constraint to get the above behavior?

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema targetNamespace="http://www.mynames.org"
xmlns:mf="http://www.mynames.org"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns="http://www.mynames.org" elementFormDefault="qualified">
<xs:element name="root">
<xs:complexType name="">
<xs:sequence>
<xs:element name="my-objects">
<xs:complexType>
<xs:sequence>
<xs:element name="object"
maxOccurs="unbounded">
<xs:complexType>
<xs:attribute name="id"
use="required"/>
<xs:attribute name="class"
use="required"/>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="object-ref" maxOccurs="unbounded">
<xs:complexType>
<xs:attribute name="id"/>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>

<!-- here are the constraints -->
<xs:key name="objectIdKey">
<xs:selector xpath=".//mfbject"/>
<xs:field xpath="@id"/>
</xs:key>
<xs:keyref name="objectRef" refer="objectIdKey">
<xs:selector xpath=".//mfbject-ref"/>
<xs:field xpath="@id"/>
</xs:keyref>
<xs:unique name="classUnique">
<xs:selector xpath=".//mfbject"/>
<xs:field xpath="@id"/>
</xs:unique>

</xs:element>
</xs:schema>


Thanks,
Olaf