I would like to get the value selected from a selection on a form then use that value on my sql query that would then populate my second selection on the form.

"""select id, name from %s where usage='the_value_from the selection' order by name asc""" %('stock_location',)

I have searched the net and looked into the codes in openerp , read about on_change method but I cannot get the proper approach to it.