one2many field get row number

I have a module contain one2many filed.

while I create data line in this o2m field, I'd like to append a row number to it.

I have try some method that I found in forum, like this link.

but since I have no function called _onchange_partner_id() , I don't know how to use it.

or this link .

but it seems like an old version method that I can't get well.

class YcWeight(models.Model):

    _name = "yc.weight"
    customer_detail_ids = fields.One2many("yc.weight.details", "name", "customer details")

class YcWeightDetails(models.Model):
    _name = "yc.weight.details"

    name = fields.Many2one("yc.weight", "weight detail list", ondelete="cascade")
    no = fields.Integer("row number")

the "no" is a field that I want to show number of row count. my problem is :

how can I get get the number of rows?

since onchage decorated function can't get data from db.

I find a solution by myself and it is simple:

use depends decorator.

class YcWeightDetails(models.Model):
    _name = "yc.weight.details"

    name = fields.Many2one("yc.weight", "weight detail list", ondelete="cascade")
    no = fields.Integer("row number")
    compuute_no = fields.Integer("invisible field", compute= "_get_row_no")

create a field "compuute_no" to compute.

    @api.depends("compuute_no")
    def _get_row_no(self):
        if self.ids:
            count =1
            for rec in self:
                weight_id = self.env['yc.weight.details'].search([('id','=', rec.id)])
                weight_id.write({'no': count})
                count+=1

or overwrite create method

@api.model
    def create(self, vals):
        main_key = self.env["yc.weight"].search([], order="id desc", limit=1).id
        item_key = vals["name"]
        if item_key and main_key == item_key:
            number = len(self.env["yc.weight.details"].search([("name", "=", item_key)]))
            vals.update({"no": number + 1})
            return super(YcWeightDetails, self).create(vals)

hope it can help you.