My XML Looks like this:

<?xml version="1.0" encoding="utf-8" ?>
<projects>
    <project 
        id="1"
        thumb="media/images/thumb.jpg"
        >
        <categories>
            <id>1</id>
            <id>2</id>
        </categories>
        <director>Director name</director>
        <name><![CDATA[IPhone commercial]]></name>
        <url><![CDATA[http://www.iphone.com]]></url>
        <description><![CDATA[Description about the project]]></description>
        <thumb><![CDATA[/upload/images/thumb.jpg]]></thumb>
    </project>
</projects>

But I cannot figure out how to filter projects based on a category id? Does anybody know how to do ? :)