selection parameter in many2one field doesn't work

[Digitel]

Version 5.0.9
When trying to use the selection parameter on a field, the result is not filtered at all.
In the example mentioned below, all extensions are present when having to chose the extension and not only the one returned by the _sel_extension function

class asterisk_user(osv.osv):

    def _sel_extension(self, cr, uid, context={}):
        obj = self.pool.get("asterisk.extensions")
        ids = obj.search(cr, uid, [('type','=','sip_extension')], context=context)
        res = obj.read(cr, uid, ids, ['name','id'], context=context)
        res = tuple([(r['id'],r['name']) for r in res])
        return res

    _name = "res.users"
    _inherit = "res.users"
    _columns = {
        "extension":fields.many2one('asterisk.configuration.file.category','extension',selection=_sel_extension),
    }
asterisk_user()