NULLs in foreign keys ok?

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(Sorry, just started using CDBI this weekend. BTW-Rock on, I love this 
thing.)

Assuming I have a class with a foreign key, but the foreign key can 
sometimes be null.  For example, if I had a Person class, and the person 
table had a employer_id which referred to an employer table, but some 
people might be unemployed, so they'd have employer_id set to NULL.

Is it still safe to set up a has_a relationship?  What happens if I call 
the employer() method that I set up in that case?  Does it return undef, 
indicating that this person, in fact, doesn't "have a" employer?  :)

-ofer