Any way to get a creative ID from an ad ID, or vice-versa?

I don't see any way in the docs to get a creative ID from an ad ID, or the other way around.

You can get a list of creative IDs given a single image ID: creatives is a field in the ad image endpoint.

But there is no ads field in the ad creative endpoint, and there is no creatives field in the ad insights endpoint.

Am I missing something?