[web2py] update with variable field (stored in variable)

[Serbitar]

I need to do something like this:

variable_field = 'val1'
x = 2
id = 1

db.define_table('a',
    Field('name', type = 'integer'),
    Field('val1', type = 'integer'),
    Field('val2', type = 'integer')
)

db(db.a.id==id).update(variable_field=x)


this obviously doesnt work.

Any way to do that?