I have inherit res.partner and added a field named: rel_zone = fields.Integer(compute="_getzone", string='Zone', readonly=False)

In _getzone there is a lookup regarding country, zipcode = postal zone.

In my custom module I'm referring to 

customer_id = fields.Many2one('res.partner', String="Customer")

zone = fields.Integer(string="Zone code", related='customer_id.rel_zone', store=True, readonly=True)

 File "C:\Bitnami\odoo-12\apps\odoo\Lib\site-packages\odoo-12.0.post20200815-py3.7.egg\odoo\fields.py", line 547, in _setup_related_full
    field = target._fields[name]
KeyError: 'rel_zone'