It works using the example many to many of the book:

for x in db(db.auth_user.id>0).select(db.auth_user.id):
    groups = []
    users_and_groups = db((db.auth_user.id==x.id)& 
(db.auth_user.id==db.auth_membership.user_id) & 
(db.auth_group.id==db.auth_membership.group_id))
    for g in users_and_groups.select(db.auth_group.role, 
orderby=db.auth_membership.group_id):
        groups.append(g.role)
    # [...]


Em quarta-feira, 2 de dezembro de 2015 14:31:14 UTC-2, Fabiano Almeida 
escreveu:
>
> Hi!
>
> I'm trying to create a list of roles for each user:
>
> for x in db(db.auth_user.id>0).select(db.auth_user.id):
> ....groups = []
> ....for g in db(db.auth_membership.id > 0).select(db.auth_group.role, 
> join=[db.auth_group.on(db.auth_group.id==db.auth_membership.group_id), 
> db.auth_user.on(db.auth_user.id==db.auth_membership.user_id)]):
> ........groups.append(g.role)
>
> and the error:
>
> TypeError: reduce() of empty sequence with no initial value
>
> How to do this list?
>
> thanks,
>
> Fabiano
>

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