I'm currently trying to have a field default to a computed value upon creation, but then remain unchanged after the record has been created. I've extended 2 models MrpWorkorder and MrpRouting to add some fields like so:

class MrpRouting(models.Model:
	​_name = "mrp.workorder"
	​_inherit = "mrp.workorder"

	​planning_bucket = fields.Integer("Planning Bucket")

class MrpWorkorder(models.Model):
	​_name = "mrp.workorder"
	​_inherit = "mrp.workorder"

	​planning_bucket = fields.Integer("Planning Bucket",
		​default=lambda self: self.operation_id.planning_bucket​
	​)

The ideal behavior im going for here, is that when a work order is created, it automatically sets the value of planning_bucket based on the planning_bucket on the associated operation. Then, once the work order is created, the value of planning_bucket should remain unchanged, meaning if i later change the planning_bucket on the operation, it wont change on the work order.

The above code is setting the planning_bucket to 0 on every work order, even if a planning_bucket value exists on the associated operation. I was able to get the values to set properly with the compute argument, but the issue i had with that was that any time the operation's planning bucket changed, the work order planning bucket also changed.

I'm running Odoo 15.0. Any help would be really appreciated!