I want to make a field Unique with combination of two fields and it does not show on form

class shipment_charge(osv.osv):
    _name = 'shipment.charge'
    _columns = {
        'type': fields.selection(list_type, 'Type', required=True, ),
        'mode': fields.selection(list_mode, 'Mode', required=True, help=""),
        'transit_time': fields.selection(list_transit_time, 'Transit Time', required=True, help=""),
        'charges_key':  fields.char('charges_key', size=64, help=""),
    }

    _sql_constraints = [
        ('charges_key_unique', 'unique (type, mode)', 'The Charges key is not Unique')
    ]