After struggling to get the right code I finally found it. The following works perfectly all the time.

I hope this helps someone.

from odoo import models, fields, api

class PurchaseOrderLine(models.Model):
    _inherit = "purchase.order.line"

    barcode_scan = fields.Char(
                   string='Product Barcode',
                   readonly=False,
                   help="Scan the product barcode to automatically set the product and quantity."
                   )
    packaging_id = fields.Many2one(
        'product.packaging',
        string='Product Packaging',
        domain="[('product_id', '=', product_id)]" # Filter packagings based on the selected product
    )
    @api.onchange('barcode_scan')
    def _onchange_barcode_scan(self):
        if not self.barcode_scan:
            return
        barcode = self.barcode_scan.strip()
        packaging = self.env['product.packaging'].search([('barcode', '=', barcode)], limit=1)
        multi_barcode = self.env['product.template.barcode'].search([('name', '=', barcode)], limit=1)
        product = self.env['product.product'].search([('barcode', '=', barcode)], limit=1)
        if packaging:
            self.product_id = packaging.product_id
            self.packaging_id = packaging # Set the packaging_id field here
            self.product_qty = packaging.qty # Set quantity directly here as a fallback
        elif multi_barcode:
            self.product_id = multi_barcode.product_id
            self.packaging_id = False
        elif product:
            self.product_id = product
            self.packaging_id = False
        else:
            self.product_id = False
            self.packaging_id = False

    @api.onchange('product_id')
    def onchange_product_id_override(self): # Use a distinct name for your overriding method
        # Call the default Odoo onchange logic first
        super(PurchaseOrderLine, self).onchange_product_id()  # Use the correct parent method name here

        # After the default logic, update the quantity based on packaging
        if self.packaging_id:
            self.product_qty = self.packaging_id.qty

    @api.onchange('packaging_id')
    def _onchange_packaging_id(self):
        """Update product_qty based on the selected packaging."""
        if self.packaging_id:
            self.product_qty = self.packaging_id.qty
        else:
            # Optional: handle when no packaging is selected
            if self.product_id:
                 # You might want to reset to the product's default purchase quantity
                 self.product_qty = 1 # Example: Set a default quantity of 1
            else:
                 self.product_qty = 0 # Or a default value

Hi,

You're on the right track, but the issue is that you're trying to set self.product_quantity, which doesn't exist on purchase.order.line. The correct field name for quantity in purchase.order.line is 'product_qty'.

Update your code as follows.

self.product_qty = package_barcode.qty

Hope it helps

Hii,

Set product_qty after product selection and after Odoo's default logic finishes.

Use @api.onchange on barcode_scan and delay setting product_qty

Override the quantity after product assignment and onchange logic like this:

from odoo import models, fields, api

class PurchaseOrderLine(models.Model):
    _inherit = "purchase.order.line"

    barcode_scan = fields.Char(
        string='Product Barcode',
        related='product_id.barcode',
        readonly=False,
        help="Here you can provide the barcode for the product."
    )

    @api.onchange('barcode_scan')
    def _onchange_barcode_scan(self):
        """Onchange function for barcode input that finds product and updates quantity."""
        if self.barcode_scan:
            # Search in multi-barcodes
            multi_barcode = self.env['product.template.barcode'].search([('name', '=', self.barcode_scan)], limit=1)
            # Search in packaging
            packaging = self.env['product.packaging'].search([('barcode', '=', self.barcode_scan)], limit=1)
            # Search in default barcode
            product = self.env['product.product'].search([('barcode', '=', self.barcode_scan)], limit=1)

            if multi_barcode:
                self.product_id = multi_barcode.product_id
            elif packaging:
                self.product_id = packaging.product_id
                # Delay setting product_qty so that _onchange_product_id doesn't override it
                self._update_quantity_after_onchange(packaging.qty)
            elif product:
                self.product_id = product

    def _update_quantity_after_onchange(self, qty):
        """Helper to delay setting quantity after default onchange logic."""
        if qty > 0:
            self.product_qty = qty

This approach ensures that product_qty is set after the default product onchange logic.
i hope this code is use full