[web2py] dependent drop down list... sometimes they come back

[Manuele Pesenti]

Hi,
Reading past posts about this topic and the book I didn't realize how to 
solve it. Imagine to have the subsequent controller called "bar"

d = dict(
        a=[1,2,3],
        b=[4,5,6])
def foo(k):
    if k in d:
        return d[k]
    else:
        l = []
        for i in d.values():
            l += i
        return l

def bar():
    form = SQLFORM.factory(
        Field('username', requires=IS_NOT_EMPTY()),
        Field('key', requires=IS_IN_SET(d.keys())),
        Field('value', requires=IS_IN_SET(foo(request.vars.key))))
    if form.accepts(request.vars, session):
        pass # or take some action
    return dict(form=form)

What's the Ajax I have to add to my view in order to modify on the fly 
the drop down list of the "value" field?

Thank you very mutch in advance
Cheers
        Manuele