[Zope] setting Zclass instance id based on properties

I would like to set the id of a Zclass instance based on some of the properties I can not use the property variables in the add method since the properties have not yet been assigned a value For instance let's say I have three properties: Section, Chapter and Page How do I assign the id the value <dtml-var Section>-<dtml-var Chapter>-<dtml-var Page> when I create the zclass instance?