In multiple Sitecore sites I have implemented a separate SQL database for storing information related to the user profile where one-to-many relationships where concerned. One such example is:
UserA has a BMW and Ford UserB has a Mercedes, Volvo and Toyota.
These values might appear in the user profile area where the user can add/delete.
Is there a best practice for this type of user profile information that is self-contained in Sitecore that I have overlooked?