XSD schema attempt to make attribute value unique

I have been trying to use xs:unique with no success so far. I have the following simple schema:

    <?xml version="1.0" encoding="utf-8"?>
    <xs:schema xmlns="http://testuri/test.xsd" 
      xmlns:xs="http://www.w3.org/2001/XMLSchema" 
      targetNamespace="http://testuri/test.xsd" 
      elementFormDefault="qualified" 
      id="XMLSchema1">
      <xs:element name="root">
        <xs:complexType>
          <xs:sequence>
            <xs:element name="items">
              &l开发者_如何学Got;xs:complexType>
                <xs:sequence>
                  <xs:element name="item" maxOccurs="unbounded">
                    <xs:complexType>
                      <xs:attribute name="id" type="xs:string" use="required"/>
                    </xs:complexType>
                  </xs:element>
                </xs:sequence>
              </xs:complexType>
            </xs:element>
          </xs:sequence>
        </xs:complexType>
      </xs:element>
    </xs:schema>

Using this the following simple XML file is validated:

    <?xml version="1.0" encoding="utf-8" ?>
    <root xmlns="http://testuri/test.xsd">
      <items>
        <item id="1"/>
        <item id="1"/>
        <item id="1"/>
      </items>
    </root>

Now let's say we want to make attribute id unique. I was thinking of simply changing the schema to:

    <?xml version="1.0" encoding="utf-8"?>
    <xs:schema 
      xmlns="http://testuri/test.xsd" 
      xmlns:xs="http://www.w3.org/2001/XMLSchema" 
      targetNamespace="http://testuri/test.xsd" 
      elementFormDefault="qualified" id="XMLSchema1">
      <xs:element name="root">
        <xs:complexType>
          <xs:sequence>
            <xs:element name="items">
              <xs:complexType>
                <xs:sequence>
                  <xs:element name="item" maxOccurs="unbounded">
                    <xs:complexType>
                      <xs:attribute name="id" type="xs:string" use="required"/>
                    </xs:complexType>
                  </xs:element>
                </xs:sequence>
              </xs:complexType>
            </xs:element>
          </xs:sequence>
        </xs:complexType>
        <xs:unique name="itemKey">
          <xs:selector xpath="items/item"/>
          <xs:field xpath="@id"/>
        </xs:unique>
      </xs:element>
    </xs:schema>

However the xml above still gets validated.

I am most certain that the problem lies on the xpath of the selector. Any ideas?

I think you must qualify the xpath expression with namespace prefixes (since XPath 1.0 has no notion of default namespace). So add a namespace declaration for your target namespace:

<xs:schema 
  xmlns="http://testuri/test.xsd" 
  xmlns:xs="http://www.w3.org/2001/XMLSchema" 
  targetNamespace="http://testuri/test.xsd" 
  xmlns:p="http://testuri/test.xsd"
  elementFormDefault="qualified" id="XMLSchema1">

and then use that prefix in your xpath expression:

    <xs:unique name="itemKey">
      <xs:selector xpath="p:items/p:item"/>
      <xs:field xpath="@id"/>
    </xs:unique>

I have not verified this, though.