[FIX] model that same pattern for _fields_sync

- add test ensure_once()

partner_contact_in_several_companies/models/res_partner.py

@@ -138,18 +138,18 @@ class ResPartner(models.Model):
         after create/update, just as if those were all modeled as
         fields.related to the parent
         """
+        self.ensure_one()
         super(ResPartner, self)._fields_sync(update_values)
-        for record in self:
-            contact_fields = record._contact_fields()
+        contact_fields = self._contact_fields()
         # 1. From UPSTREAM: sync from parent contact
         if update_values.get('contact_id'):
-                record._contact_sync_from_parent()
+            self._contact_sync_from_parent()
         # 2. To DOWNSTREAM: sync contact fields to parent or related
         elif any(field in contact_fields for field in update_values):
-                update_ids = record.other_contact_ids.filtered(
+            update_ids = self.other_contact_ids.filtered(
                 lambda p: not p.is_company)
-                if record.contact_id:
-                    update_ids |= record.contact_id
+            if self.contact_id:
+                update_ids |= self.contact_id
             update_ids.update_contact(update_values)
 
     @api.onchange('contact_id')