Re: one-to-one schema design question and ORM

On Fri, Mar 09, 2007 at 10:06:52 -0500,
  Rick Schumeyer <rschumeyer@ieee.org> wrote:
>
> From a business rules perspective:
>   Some users are not employees (like an admin user)
>   Some employees are not users
>
> I can think of two ways to do this:
>
> 1) a 1-1 relationship where the user table contains a FK to the employee
> table.  Since not all users will be employees, the FK will sometimes be
> null.
> In rails, the user class would "belong_to employee" while employee
> "has_one user".
>
> 2) Create a link table that has FKs to both the user and employee
> table.  This make sense because I'm not sure that the concept of "there
> might be a linked employee" belongs in the user table.  This moves it to
> a separate table designed for that purpose.  But then again, it may just
> be a needless extra table.
>
> Would you prefer one solution over the other?

I think you need a linking table to properly represent the business rule
above. You can use unique constraints on each key in the link table,
to enforce a 1 to 1 link for the users that are employees.