我试图用pos.order.line模型中调用的以下函数更新一个2many字段，因为“tax\u ids\u after\u fiscal\u position”字段没有保存问题。这是我的代码：

class PosOrderLine(models.Model):
    _inherit = "pos.order.line" 

    @api.onchange('product_id')
    def _onchange_product_id(self):
        self.tax_ids_after_fiscal_position = self.product_id.taxes_id
        self.write({'tax_ids_after_fiscal_position': [(4, self.product_id.taxes_id)]})

我得到以下错误：

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/odoo/http.py", line 639, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "/usr/lib/python3/dist-packages/odoo/http.py", line 315, in _handle_exception
    raise exception.with_traceback(None) from new_cause
AttributeError: 'account.tax' object has no attribute 'ref'

请问怎么了？有什么帮助吗？谢谢。