Complicated (?) Access to Records

Here's my problem. A user logs into a FileMaker DB through a Custom Web interface (using PHP, and FX). A user is able to look at information about some subset of rooms, depending on access set up by an administrator. For each user, there may be one or more privileges, for example, user A may look at rooms in Bldg XYZ, while another user B may look at rooms in dept 123. User C could look at rooms for bldg QRS *and* dept 199.

I'm assuming that I'll have to have something like this set of tables/fields:

User: UserID Name

Access: UserID Bldg Dept

Room: Bldg RoomNum Dept RoomData

A user will only be able to see data from rooms for which she has access, either because she has access to the building or the department.

A user can have access to more than one building. Or even the intersection of a building and a department.

It seems like the room table might be a good place for an unstored calculation field which would calculate whether the current user (perhaps using a global field) should have access to this record. Then from the interface, I would include that field (=true) in the search criteria.

With multiple users, I would need to make sure that the field is session specific.

I realize the answer may be quite complicated, but I'm not sure what direction to look in.

[i don't think that the FileMaker built in accounts and privileges setup will work, partly because the situation is even more complicated, with at least three levels of privilege (read only, read/write, and nothing), but ignore that for now... plus another dimension I don't even want to think about.]

Thanks,

Hairless in Ithaca