Maybe something like this? (not tested)
already_owns = db(db.owns.person == person_id)._select(db.owns.thing)
things_not_owned = db(~db.thing.id.belongs(already_owns)).select()
-Jim
On Thursday, March 10, 2016 at 8:49:32 AM UTC-6, Paul wrote:
>
> Does anyone know if there is a way to perform set difference (ie the
> elements in set A that are not in set B) using database queries / DAL?
>
> for example if you had some tables defined like this:
>
> db.define_table('thing',
> Field('name'))
>
> db.define_table('owns',
> Field('person', 'reference auth_user'),
> Field('thing', 'reference thing'))
>
>
> and you want to find a list of all the things that person X doesn't yet
> own.
>
> The "easy" way would be to query for a set of all the "thing"s and then do
> a separate query for all the "thing"s owned by person X, create python set
> objects and use the set difference operator.
>
>
> I'm wondering if there is a way to do this as a single db query?
>
> Thanks!
>
> Paul
>
>
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