[Zope] ZODB: When to use IDs when references?

Up to now I used relational databases. Now I don't know when it is better to use IDs and when to use references. Background: I write a Multiple Choice Python Product. There is a list of questions and a list of users. Both get combined in the "result" class. Should the class "result" have references to the questions or store the question-IDs? Any ideas welcome! -- Open Source Software Solutions Thomas Guettler <thomas@thomas-guettler.de> http://www.thomas-guettler.de