I played around and came up with this:

```ruby
receiver = ExternalServiceReceiver.find_or_initialize_by_receiver_id_and_external_service_id od.id, external_service.id
receiver.response_data = response_data
receiver.save!
```
I think that does what you want and avoids the conditional. The tests all pass. If there are validations/callbacks it might not work exactly the same since you're using `#update_attribute`.