Hi All,

        I have written a simple schema (see below) that uses substitution
groups.  I don't know if this the correct usage because I'm fairly new to
xml schema.

The structure is as follows:

Request
        Report
                TargetReference (reference identifier attribute)
        Target (identifier attribute)

Example Document:

<?xml version="1.0" encoding="UTF-8"?>
<Request xmlns="urn:demo"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="urn:demo
Untitled23.xsd">
        <DummyReport identifier="10">
                <TargetReference identifier="20"/>
        </DummyReport>
        <DummyTarget identifier="20"/>
</Request>

A valid document can contain a number of reports and targets.  The target
reference element should reference only targets in the document and I would
like to enforce this.  Does anybody know how to do this?  I was initially
thinking along the lines of using the Key and KeyRef elements but don't know
how to do that with substitution groups?  Is it possible?  I would also like
to enforce that all target identifier attributes are unique.  Can this be
done?

Any help would be appreciated.

Regards

Darren

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

        <!-- Root element request -->
        <xs:element name="Request">
                <xs:complexType>
                        <xs:sequence>
                                <xs:element ref="ReportGroup"
maxOccurs="unbounded"/>
                                <xs:element ref="TargetGroup"
maxOccurs="unbounded"/>
                        </xs:sequence>
                </xs:complexType>
        </xs:element>

        <!-- Abstract report type -->
        <xs:complexType name="Report" abstract="true">
                <xs:sequence>
                        <xs:element ref="TargetReference"
maxOccurs="unbounded"/>
                </xs:sequence>
                <xs:attribute name="identifier" type="xs:int"
use="required"/>
        </xs:complexType>

        <!-- Abstract target type -->
        <xs:complexType name="Target" abstract="true">
                <xs:attribute name="identifier" type="xs:int"
use="required"/>
        </xs:complexType>
        <xs:element name="ReportGroup" type="Report" abstract="true"/>
        <xs:element name="TargetGroup" type="Target" abstract="true"/>

        <!-- Element to join a report to a target -->
        <xs:element name="TargetReference">
                <xs:complexType>
                        <xs:attribute name="identifier" type="xs:int"
use="required"/>
                </xs:complexType>
        </xs:element>

        <!-- Reports -->
        <xs:element name="DummyReport" substitutionGroup="ReportGroup">
                <xs:complexType>
                        <xs:complexContent>
                                <xs:extension base="Report"/>
                        </xs:complexContent>
                </xs:complexType>
        </xs:element>
        <xs:element name="AnotherDummyReport"
substitutionGroup="ReportGroup">
                <xs:complexType>
                        <xs:complexContent>
                                <xs:extension base="Report"/>
                        </xs:complexContent>
                </xs:complexType>
        </xs:element>

        <!-- Targets -->
        <xs:element name="DummyTarget" substitutionGroup="TargetGroup">
                <xs:complexType>
                        <xs:complexContent>
                                <xs:extension base="Target"/>
                        </xs:complexContent>
                </xs:complexType>
        </xs:element>
        <xs:element name="DummyTarget2" substitutionGroup="TargetGroup">
                <xs:complexType>
                        <xs:complexContent>
                                <xs:extension base="Target"/>
                        </xs:complexContent>
                </xs:complexType>
        </xs:element>

</xs:schema>