Hi,

If you want to compute a field in the child model based on a field in the parent model, you can use related function field that computes the value based on the parent model's field.
Here's an example using related:
from odoo import models, fields, api

class PurchaseOrder(models.Model):
    _inherit = 'purchase.order'

    # Add a related field to the child model
    parent_product_qty = fields.Float(related='order_line.product_qty', string='Parent Product Quantity')

   @api.depends('parent_product_qty')

    def _hellooo(self):

         for order in self:

             for line in order.order_line:

                  a=1



Hope it helps