Foreign Key column mapped to multiple primary keys

Question

I have a database which has three tables

Messages - PK = MessageId
Drafts - PK = DraftId
History - FK = RelatedItemId

The History table has a sin

Answer 1

Is there a name for this relationship?

There is no standard name that I'm aware of, but I've heard people using the term "generic FKs" or even "inner-platform effect".

Is it just bad design?

Yes.

The reason: it prevents you from declaring a FOREIGN KEY, and therefore prevents the DBMS from enforcing referential integrity directly. Therefore you must enforce it trough imperative code, which is surprisingly difficult.

Is there a better way to design this relationship?

Yes.

Create separate FOREIGN KEY for each referenced table. Make them NULL-able, but make sure exactly one of them is non-NULL, through a CHECK constraint.

Alternatively, take a look at inheritance.

Answer 2

In a short description the solution you have used is called:
Polymorphic Association
Objective: Reference Multiple Parents
Resulting anti-pattern: Use dual-purpose foreign key, violating first normal form (atomic issue), loosing referential integrity
Solution: Simplify the Relationship

More information about the problem.

BTW createing a common super-table will help you:

Answer 3

Best practice I have found is to create a Function that returns whether the passed in value exists in either of your Messages and Drafts PK columns. You can then add a constraint on the column on the History that calls this function and will only insert if it passes (i.e. it exists).

Adding non-parsed example Code:

CREATE FUNCTION is_related_there ( IN @value uniqueidentifier ) RETURNS TINYINT BEGIN IF (select count(DraftId) from Drafts where DraftId = @value + select count(MessageId) from Messages where MessageId = @value) > 0 THEN RETURN 1; ELSE RETURN 0; END IF; END;

ALTER TABLE History ADD CONSTRAINT CK_HistoryExists CHECK (is_related_there (RelatedItemId) = 1)

Hope that runs and helps lol