I've almost found a proper way, but it still has some issues

With the following code inside `sale_order._onchange_order_line()` I could create multiple new records (saved into the DB) and add them into user's editing tree view

class SaleOrder(models.Model):
  _inherit = 'sale.order'

  @api.onchange('order_line')
  def _onchange_order_line(self):
    # store here new lines that we might create below (we use an empty 'sale.order.line' recordset as default value in order to easily add records to it as we need ...)
    new_lines = self.env['sale.order.line']

    # if we need to add one or multiple sale order lines in the DB
    new_line = self.order_line.create({
      'order_id': self._origin.id,
      'product_id': 10, # some product.product ID,
    })

    # trigger some methods to update all the rest of the values
    new_line.product_id_change()
 
    # add new line to the new_lines recordset
    new_lines |= new_line

    # some other processing, even adding couple of more lines like above ...

    # at the end, in case we need to send back the newly created lines ...
    if new_lines:
      # combine existing order_line with the new lines
      all_lines = self.order_line + new_lines

      return {
        'value': {
          'order_line': all_lines
        }
      }

But, the new problems I face now, are:

• if I remove any of this newly created records which I'm injecting into the view (before save) and then save, it's looking ok (they're removed from the view) except that they're NOT actually deleted from the DB (if I save or reload they get back in the view)

• if I discard the changes all these records are removed from the view which normally it shouldn't as they're save into the DB (not phantom records) and the user get's confused but actually they're still in the DB (on refresh they get back into the view)

If I manually save the order as soon as those lines are added and edit it back everything it's ok

So I am very close but still missing a small detail I guess ...

Same result with the same issues I have also if I just connect the new records like this (instead of returning the new combined lines like above):

self.order_line = [(4, ln.id, 0) for ln in new_lines]

.