Ah, sorry, I didn't notice that this was an instance method.

However, seeing that example, I'm not a big fan of the duplication of information. `id_number` and `user_id` now exist in multiple places: `+managedObjectKeysByPropertyKey` and this method.

Maybe it'd be nice to offer a method which indicates which keys should participate in uniquing, like `+propertyKeysForManagedObjectUniquing`. Then the adapter can read those values off the instance directly, and the Core Data keys don't have to be specified again.