[xml] XML Schema, unique and namespaces

[allan awn dk]

Hey,

I'am trying to write a xml schema which must validate that a given
attribute is unique.

A (valid) simple example of the problem could look like this:
<root xmlns="http://ns2"; >
  <a id="a/1" />
  <a id="a/2" >
    <a id="a/3" />
    <a id="a/4"/>
  </a>
</root>

An invalid example (where the ID field is not unique) could look like this:
<root xmlns="http://ns2"; >
  <a id="a/1" />
  <a id="a/2" >
    <a id="a/1" />
    <a id="a/4"/>
  </a>
</root>

The schema I have been experimenting with looks like this:

<xsd:schema
  xmlns:xsd="http://www.w3.org/2001/XMLSchema";
  xmlns:ptt="http://ns2";
  targetNamespace="http://ns2";
  elementFormDefault="qualified" >

  <xsd:element name="root" type="ptt:myList">
    <xsd:unique name="myId">
      <xsd:selector xpath="ptt:*"/>
      <xsd:field xpath="@id"/>
    </xsd:unique>
  </xsd:element>

  <xsd:complexType name="myList">
    <xsd:sequence minOccurs="1">
      <xsd:element name="a" minOccurs="1" maxOccurs="unbounded"
type="ptt:asdf" />
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="asdf">
    <xsd:sequence minOccurs="1">
      <xsd:element name="a" minOccurs="0" maxOccurs="unbounded"
type="ptt:asdf" />
    </xsd:sequence>
    <xsd:attribute name="id" type="xsd:string"/>
  </xsd:complexType>
</xsd:schema>

I belive my problem is, that I can not figure out the correct xpath to
write in the
selector.

Best regards
Allan W. Nielsen