You're having that problem because you're trying to deal with an implementation issue while still in the design phase.

Don't worry about HOW you'll implement something until you know WHAT you'll be dealing with. About the only time that sort of thing arises at design time is when the designer is doing a cost/benefit analysis of a particular feature. In such a case, the designer relies on experience to gauge what is feasible and how dificult it may be to implement