Insert

Controller

@PostMapping
public ResponseEntity<UserDTO> insert(@RequestBody UserDTO userDTO) {
    userDTO = userService.insert(userDTO);
    URI uri = ServletUriComponentsBuilder.fromCurrentRequest().path("/{id}").buildAndExpand(userDTO.getId()).toUri();
    return ResponseEntity.created(uri).body(userDTO);
}

Service

public UserDTO insert(UserDTO userDTO) {
    User user = new User();
    copyDtoToEntity(userDTO, user);
    //agora usar insert para o mongoDB, não save
    user = userRepository.insert(user);
    return new UserDTO(user);
}