11.0

models/purchase.py

@@ -109,6 +109,12 @@ class PurchaseOrder(models.Model):
             vals['name'] = self.env['ir.sequence'].next_by_code('opened.purchase.order') or '/'
         return super(PurchaseOrder, self).create(vals)
 
+    @api.multi
+    def unlink(self):
+        if self.is_opened:
+            self.button_cancel()
+        return super(PurchaseOrder, self).unlink()
+
 class ProcurementGroup(models.Model):
     _inherit = "procurement.group"