you can do that with
>
> all_courses = db(db.courses.id>0)._select(db.courses.memberid)
> all_members_not_in_courses = db(~db.members.id.belongs(all_courses))
>
>
>>
Would such an approach be computationally expensive particularly if the
number of rows in courses is high? Perhaps the following raw SQL expression
would be more efficient:
from gluon.dal import Expression
all_members_not_in_courses = db((db.members.id>0)&Expression('NOT EXISTS
(SELECT courses.memberid FROM courses WHERE
courses.memberid=members.id)')).select()
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