XML Schema - Unique attribute value + parent attribute value

Wanting to write a schema that tests to see if the combination of an attribute + the parent node attribute is unique.

Have the following so far

<xs:unique name="specify_req_once_in_traceability_section"开发者_运维问答>
    <xs:selector xpath="./artefact/doc/relationship"/>
<xs:field xpath="@parent_sec"/>
</xs:unique>    

but if I try to include the parent_doc using .. I get unexpected token --- I am guessing the parent axis is not allowed

xml looks like below

<?xml version="1.0" encoding="UTF-8"?>
    <root>
        <artefact>
             <doc parent_doc="fred">
                   <relationship parent_sec="125"/>          
             </doc>
             <doc parent_doc="geoff">
                   <relationship parent_sec="119"/>
                   <relationship parent_sec="118"/>
                   <relationship parent_sec="117"/> 
                   <relationship parent_sec="118"/> 
             </doc>
             <doc parent_doc="fred">
                   <relationship parent_sec="125"/>
             </doc>
             <doc parent_doc="james">
                   <relationship parent_sec="125"/>
              </doc>
          </artefact>
    </root>

I tested my hypothesis in the comment and it works using the xerces version in JDK 6. I simplified the schema a bit, but the general structure is the same:

Schema

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="root">
    <xs:complexType>
        <xs:sequence>
            <xs:element name="doc" maxOccurs="unbounded">
                <xs:complexType>
                    <xs:sequence>
                        <xs:element name="relationship" maxOccurs="unbounded">
                            <xs:complexType>
                                <xs:attribute name="ps" />
                            </xs:complexType>
                        </xs:element>
                    </xs:sequence>
                    <xs:attribute name="pd" />
                </xs:complexType>
            </xs:element>
        </xs:sequence>
    </xs:complexType>
    <xs:unique name="u1">
        <xs:selector xpath="doc"/>
        <xs:field xpath="@pd"/>
        <xs:field xpath="relationship/@ps" />
    </xs:unique>
</xs:element>
</xs:schema>

Instance document

<root>
<doc pd="fred">
    <relationship ps="125" />
</doc>
<doc pd="geoff">
    <relationship ps="119" />
    <relationship ps="118" />
    <relationship ps="117" />
    <relationship ps="118" />
</doc>
<doc pd="fred">
    <relationship ps="125" />
</doc>
<doc pd="james">
    <relationship ps="125" />
</doc>
</root>

When validated, xerces give error message:

[Error] :13:28: Duplicate unique value [fred,125] declared for identity constraint of element "root".

If you change the second james' relationship ps to 126, the document validates without errors.