/**
   * Adds an association between a challenge and a challenge referral.
   *
   * @param id challenge id
   * @param challengeReferralId challenge referral id
   * @return ServiceResponse with appropriate status
   * @throws ObjectNotFoundException If one of the objects were not found
   */
  @RequestMapping(value = "/{id}/challengeReferral", method = RequestMethod.POST)
  public @ResponseBody ServiceResponse addChallengeReferralToChallenge(
      @PathVariable final UUID id, @RequestBody @NotNull final UUID challengeReferralId)
      throws ObjectNotFoundException {

    final Challenge challenge = service.get(id);
    final ChallengeReferral referral = challengeReferralService.get(challengeReferralId);

    service.addChallengeReferralToChallenge(referral, challenge);

    return new ServiceResponse(true);
  }