I use customized Workorders in my model but want to keep the base sales orders for retail etc.

So I used  _inherits in my model and it looked very promising i.e. in the beginning.

Here what I did...

class YachtService(models.Model):
    _name = 'yacht.service'
    _description = 'Workorder Data'
    _inherits = {'sale.order': 'partner_id'}
    partner_id = fields.Many2one( 'sale.order', ondelete='cascade', required='false', string='Customer')

    my_field_2 = fields.Boolean(string="Is MyField 2")

My view looks like this:

<record id="work_order_form" model="ir.ui.view">
        <field name="name">workorder.form</field>
        <field name="model">yacht.service</field>
        <field name="inherit_id" ref="sale.view_order_form"/> 
        <field name="arch" type="xml">

            <field name="payment_term_id" position="after">
                <field name='my_field_2'/>
            </field>
        </field>
    </record>

When opening my work_order_form view it all appears just like I want it to. As desired, the original sale.order.form is not modified, so far so good....

Here the problem:

In my new view, the moment I enter data into the customer field (partner_id) I get the following error.

Error:
Odoo Server Error

Traceback (most recent call last):
>
>
>
    raise ValueError("Invalid field %r in leaf %r" % (left, str(leaf)))
ValueError: Invalid field 'customer' in leaf "<osv.ExtendedLeaf: ('customer', '=', True) on sale_order (ctx: )>"

To be honest, I don't even know where to start with this problem. From what I can read, it seemingly has something to do with the validity of the Many2One field (partner_ID).