Version: Odoo 12 CE

I use Janeesh method to inherit the sale.order.line.

The modified code:

class Materials(models.Model):
    _inherit = 'yachtservice.checkin'

    material_line_ids = fields.One2many('sale.order.line', 'yachtservice_checkin_id', 'Order Lines')

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

    yachtservice_checkin_id = fields.Many2one('yachtservice.checkin', 'Materials')

So far so good. In doing so I receive inside my yachtservice.checkin model exactly the line I was hoping for.

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1. flaw I discovered, the unit price is not displayed.
2. when I try to safe I get the following error message:


The operation cannot be completed, probably due to the following:
- deletion: you may be trying to delete a record while other records still reference it
- creation/update: a mandatory field is not correctly set

[object with reference: order_id - order.id] 

Somehow I cannot find a cause for this error.

I Noticed that order_id is a field within the sale.order.line model