term = self.pool.get('product.product.terms').search(cr, uid,
                    [   ('product_id', '=', res.id),
                        ('min_quantity', '<=', qty),
                        '|', '|', ('partner_id', '=', part.id), ('partner_id', '=', None if not part.parent_id else part.parent_id.id), ('partner_id', '=', None),
                        '|', ('group_id', '=', part.calc_group_id()), ('group_id', '=', None),
                        '|', ('max_quantity', '>=', qty), ('max_quantity', '=', -1)
                    ], context=context, limit=1)