How to implement getElementById() with Xerces using XML Schema?

[Gerd Rosarius]

According to the Xerces Java API (Release 2.0.1)it should be possible to search an entire DOM-Document for an Element with a specific ID using getElementById( String elementId ).
The documentation states that I have to tell the DOM-Implementation what the name of the id-attribute is. Either by use of a DTD or a XML Schema.
If I use a DTD to specify my id-attribute and assign it to my XML file, getElementByID("123") returns an object of type Element. So far, so good... If I use a XML Schema I get 'null' instead of the expected Element.
The FAQ-Page of the Apache-Group says about Xerces:
"According to the XML Schema specification, an instance document might have more than one validation root and ID/IDREFS must be unique only within the context of a particular validation root, meaning that a document may potentially contain multiple identical ids. In this case, the output of getElementById() is unspecified. On the other hand, if the document root is a validation root of the document, getElementById() should work as expected." http://xml.apache.org/xerces2-j/faq-general.html#faq-5
Haeh???
How do I have to change my XML-Schema to get an Element instead of 'null' by use of getElementById()?
Here is the XML- and Schema-code which I tried. Any help is welcome!
H.-Gerd Rosarius

The XML file:
-------------

<?xml version="1.0" encoding="UTF-8"?>
<katalog xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="catalog.xsd">
  <book id="123">
    <author>Steven King</author>
  </book>
  <book id="124">
    <author>Tom Clancy</author>
  </book>
</katalog>

The XML Schema:
---------------

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified">
<xs:element name="katalog">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="book" maxOccurs="unbounded">
        <xs:complexType>
          <xs:sequence>
    <xs:element name="author" type="xs:string"/>
          </xs:sequence>
          <xs:attribute name="id" type="xs:NMTOKEN" use="required"/>
        </xs:complexType>
      </xs:element>
    </xs:sequence>
  </xs:complexType>
  <xs:key name="schluessel">
    <xs:selector xpath=".//book"/>
    <xs:field xpath="@id"/>
  </xs:key>
</xs:element>
</xs:schema>

[ August 27, 2002: Message edited by: Gerd Rosarius ]

[Gerd Rosarius]

Well I found the solution by my own. (Ok, ok, with a little help of a nice lady from IBM Toronto ).
Here it is:
You need to turn on both validation and schema validation features on, otherwise the getElementById() would not work correctly.
You can do that by adding the following Java code to your parser before you use the parse() method:
parser.setFeature("http://xml.org/sax/features/validation", true);
parser.setFeature("http://apache.org/xml/features/validation/schema", true);
Voil�
H.-Gerd Rosarius