How to make an element value unique within entire instance document?

[Robert Dang]

Hi, please take a look at this schema and tell me how I can make the value of my AttachmentKey element(s) unique within the entire instance document.

I know that I have to use <key> or <unique>, but I can't get it to work, as I don't have much knowledge of xpath.

here is my schema:

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema targetNamespace="test.ns.abc">
<xsd:element name="root">
<xsd:complexType>
<xsd:sequence>
<xsd:element ref="ns:A"/>
<xsd:element ref="ns:Attachment" maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:element name="Attachment">
<xsd:complexType>
<xsd:sequence>
<xsd:element ref="ns:AttachmentKey"/>
<xsd:element ref="ns:AttachmentName"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:element name="AttachmentText" type="xsd:string"/>
<xsd:element name="AttachmentName" type="xsd:string"/>
<xsd:element name="AttachmentPageTotal" type="xsd:int"/>
<xsd:element name="A">
<xsd:complexType>
<xsd:sequence>
<xsd:element ref="ns:B"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:element name="B">
<xsd:complexType>
<xsd:sequence>
<xsd:element ref="ns:Attachment"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
<xsd:element name="AttachmentKey"/>
</xsd:schema>

As you can see, AttachmentKey is a subelement of Attachment, and is used in more than one place in the example above. I want it so that when the instance document is created, no matter how many instances of Attachment is in it, each AttachmentKey value is unique, so that I can use them as keys.

So this following instance doc would be invalid:

<root>
<A>
<B>
<Attachment>
<AttachmentKey>1</AttachmentKey>
<AttachmentName>Name</AttachmentName>
</Attachment>
</B>
</A>
<Attachment>
<AttachmentKey>1</AttachmentKey>
<AttachmentName>Name</AttachmentName>
</Attachment>
<Attachment>
<AttachmentKey>2</AttachmentKey>
<AttachmentName>Name</AttachmentName>
</Attachment>
</root>

But this one would be valid:

<root>
<A>
<B>
<Attachment>
<AttachmentKey>1</AttachmentKey>
<AttachmentName>Name</AttachmentName>
</Attachment>
</B>
</A>
<Attachment>
<AttachmentKey>2</AttachmentKey>
<AttachmentName>Name</AttachmentName>
</Attachment>
<Attachment>
<AttachmentKey>3</AttachmentKey>
<AttachmentName>Name</AttachmentName>
</Attachment>
</root>

[ July 26, 2004: Message edited by: Robert Dang ]
[ July 26, 2004: Message edited by: Robert Dang ]