[web2py] how to select a row that isn't linked by a second table?

[monotasker]

I have two tables (db.A and db.B) and db.B has a field that links to db.A 
(in db.B model: Field('link', 'db.A')). How do I select rows from db.A that 
do *not* have a corresponding db.B.link field?

I've tried to solve this from several angles and so far have had no luck. I 
suspect I'm missing something obvious.

Thanks