I inherit the model stock.move and add serial_no in move lines.That field is readonly

class StockMove(models.Model):

    _inherit = "stock.move" 

   serial_no = fields.Char(string="Serial#",readonly=True)

• I use the onchange function to pass value from product

function

@api.onchange('product_id')

def on_change_product(self):

        self.serial_no = self.product_id.serial_no

I override the create function to write that value

create function

    @api.model

    def create(self, vals):

        product_obj = self.env['product.product']

        product_records = product_obj.search([])

        for record in product_records:

         self.serial_no = record.serial_no

        res = super(StockMove, self).create(vals)

        return res

notes

When i save the record then there is an error occured like this

raise ValueError("Expected singleton: %s" % self)
ValueError: Expected singleton: stock.move()

How to solve this issue ?