if I use the revision_id of the budget I would be forced to create new budgetItems for each revision of the Budget, though I actually might only change attributes of Budget (generating a new revision), but keep the budgetitems as they were before.
And then I would either have to duplicate the BudgetItems, as I can't have multiple parent_ids. Maybe I should have used acs_rels where a BudgetItem's revision is related to a Budget revision, this way allowing for an n:m relationship.
Thanks for your quick reply Dave.