visitor.partner is a many2one to partner. But partner.visitor_ids is a many2many. The result is when you set the partner_id on visitor, the (what should be) inverse field is not set accordingly. This inverse field should be a one2many fields. This commit fixes the field definition. Task ID: 2127691 PR #40577 Related: odoo/upgrade#905 Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>