Implementing RequestMethod.PATCH in Spring RestController - mongodb

Question

I am creating a Rest API for a MongoDB database using MongoRepository. I want to create an endpoint that uses "RequestMethod.PATCH" and implements the "PATCH" functionality: delta update with fields provided in the #RequestBody.
The functionality that I want already exists in "Spring Data Rest" by using the "#RepositoryRestResource" annotation on my Repository class as described here https://spring.io/guides/gs/accessing-data-rest/
But I don't want to expose my Repository class like that. I like the classic Controller->Service->Repository lineage. My controller looks like this:
#RestController
public class ActivitiesController {
#Autowired
ActivitiesService activitiesService;
#RequestMapping(value="activities", method=RequestMethod.PATCH)
public ActivityModel updateActivity(
#RequestBody ActivityModel activityModel
){
//Input ActivityModel will only have subset of fields that have been changed, aka the delta
return activitiesService.update(activityModel);
}
#RequestMapping(value="activities", method=RequestMethod.PUT)
public ActivityModel updateActivity(
#RequestBody ActivityModel activityModel
){
//Input ActivityModel will have all fields populated
return activitiesService.save(activityModel);
}
}
And my repository is here:
#Repository
public interface ActivitiesRepo extends MongoRepository<ActivityModel, String> {
//out of the box implementation
}
My problem is that, from what I can tell, MongoRepository does not provide delta updates out of the box the way that Spring Data Rest does. How can I implement that functionality in the Service layer here?:
#Service
public class ActivitiesService {
#Autowired
ActivitiesRepo activitiesRepo;
public ActivityModel update(ActivityModel activityModel){
//delta update implementation, aka PATCH implementation
}
//method that should only be used with RequestMethod.PUT
public ActivityModel save(ActivityModel activityModel){
return activitiesRepo.save(activityModel);
}
}
What do you think of this solution for a manual "PATCH" implementation:
public class ModelUtil {
public static <T> Object update(Object origModel, Object dirtyModel, Class<T> clazz){
ObjectMapper m = new ObjectMapper();
HashMap<String, Object> origModelAsMap = m.convertValue(origModel, new TypeReference<Map<String, Object>>() {});
HashMap<String, Object> dirtyModelAsMap = m.convertValue(dirtyModel, new TypeReference<Map<String, Object>>() {});
dirtyModelAsMap.forEach((k, v)-> {
origModelAsMap.put(k, v);
});
return m.convertValue(origModelAsMap, clazz);
}
}