Question

I am grappling with a database design issue at the moment, I will present a simplified scenario of what is going on:

Lets say I have four tables, Equipment, CurrentState, StateValue, StateType with the following schemas:

Equipment
------------
Id (PK),
Name


CurrentState
------------
Id (PK),
EquipmentId (FK) (IX),
StateValueId (FK),
StateTypeId (FK) (IX)


StateValue
------------
Id (PK),
StateTypeId (FK),
Name


StateType
-----------
Id (PK),
Name

A piece of equipment can have several different CurrentStates belonging to different StateTypes, hence the Unique Index (IX). A StateType is basically a state machine and StateValue contains the values for each state machine.

Now, my question, well 2 questions, is around the StateTypeId foreign keys in the CurrentState and StateValue tables that determine what StateType a CurrentState record is and what StateType a StateValue record is.

First of all, is having this type of relationship bad design in terms of referential integrity? My guess is that it IS bad design as there should only be one link between the CurrentState table and the StateType table, and this is via the StateValue, else a CurrentState record could end up having two different StateTypes (one via the direct FK and the other via the StateValue tables FK)…

But the second question comes: If I should not have a StateTypeId FK in the CurrentState table, how can I enforce the Index, that is ensure that there is not two CurrentState records for a single EquipmentId that have StateValueIds pointing to StateValue records of the same StateType…

Do I have to use a trigger when inserting into the CurrentState table to check that the rules are followed? I have never used triggers before, so I will need to do a bit of research. I am also using Entity Framework and have no idea what the implications are with this (there shouldn’t be any in theory).

Answer 1

Yes, this is bad practice, since you can have two different State Types for one current value. Drop the StateTypeId column out of CurrentState, and have it only go through StateValueId. Your unique index should be on CurrentState and StateValueId.

Of course, if you insisted on having StateTypeId in CurrentState, you could set up a check constraint that would make sure that StateTypeId was equal to the StateTypeId from StateValueId. But this is really roundabout. It’s much cleaner and more maintainable to leave them all in their own silos.