diff --git a/addons/event_sale/models/event_registration.py b/addons/event_sale/models/event_registration.py
index 999ae7ef2f1..6ff16fa1c31 100644
--- a/addons/event_sale/models/event_registration.py
+++ b/addons/event_sale/models/event_registration.py
@@ -10,8 +10,8 @@ class EventRegistration(models.Model):
 
     is_paid = fields.Boolean('Is Paid')
     # TDE FIXME: maybe add an onchange on sale_order_id
-    sale_order_id = fields.Many2one('sale.order', string='Source Sales Order', ondelete='cascade')
-    sale_order_line_id = fields.Many2one('sale.order.line', string='Sales Order Line', ondelete='cascade')
+    sale_order_id = fields.Many2one('sale.order', string='Source Sales Order', ondelete='cascade', copy=False)
+    sale_order_line_id = fields.Many2one('sale.order.line', string='Sales Order Line', ondelete='cascade', copy=False)
     payment_status = fields.Selection(string="Payment Status", selection=[
             ('to_pay', 'Not Paid'),
             ('paid', 'Paid'),