Like the idea.  One question.  What happens when you send in an invalid ID of the resource to update and no fields need to be updated?  Am I right that in this case the code completes without any notification about the non-existent resource?  If so, this is indeed a desirable behavior, but since it's different from when you do update the fields, it could be unexpected.  Perhaps adding brief comments to explain the expected behavior of Lazy and _update_dto might be helpful.