stock.picking的sale_id无法调用的问题

[billchen0769]

from odoo import api, fields, models, _

class StockPickingInherit(models.Model):
_inherit = "stock.picking"

x_partner_ref = fields.Char(string='流水号', compute='_compute_partner_ref', store=True)


@api.depends('purchase_id.partner_ref', 'sale_id.x_serial')
def _compute_partner_ref(self):
    for picking in self:
        if picking.purchase_id:
            picking.x_partner_ref = picking.purchase_id.partner_ref
        elif picking.sale_id:
            picking.x_partner_ref = picking.sale_id.x_serial
        else:
            picking.x_partner_ref = False

在上面代码中会报这个错误：
ValueError: Wrong @depends on '_compute_partner_ref' (compute method of field stock.picking.x_partner_ref). Dependency field 'sale_id' not found in model stock.picking.
但是用下面这个方法就没问题：

@api.depends('purchase_id.partner_ref')
def _compute_partner_ref(self):
    for picking in self:
        if picking.purchase_id:
            picking.x_partner_ref = picking.purchase_id.partner_ref
        else:
            picking.x_partner_ref = False

是不是在stock.picking中需要扩展sale_id字段？请问应该如何改造？