CandidateController.java

@RabbitListener(queues = "voter.rpc.requests")
private String getCandidatesMessageRpc(String requestMessage) {
    logger.debug("Request message: {}", requestMessage);
    logger.debug("Sending RPC response message with list of candidates...");

    List<CandidateVoterView> candidates = getByElection(requestMessage);

    return serializeToJson(candidates);
}

private List<CandidateVoterView> getByElection(String election) {
    Aggregation aggregation = Aggregation.newAggregation(
            Aggregation.match(Criteria.where("election").is(election)),
            project("firstName", "lastName", "politicalParty", "election")
                    .andExpression("concat(firstName,' ', lastName)")
                    .as("fullName"),
            sort(Sort.Direction.ASC, "lastName")
    );

    AggregationResults<CandidateVoterView> groupResults
            = mongoTemplate.aggregate(aggregation, Candidate.class, CandidateVoterView.class);

    return groupResults.getMappedResults();
}

private String serializeToJson(List<CandidateVoterView> candidates) {
    ObjectMapper mapper = new ObjectMapper();
    String jsonInString = "";

    final Map<String, List<CandidateVoterView>> dataMap = new HashMap<>();
    dataMap.put("candidates", candidates);

    try {
        jsonInString = mapper.writeValueAsString(dataMap);
    } catch (JsonProcessingException e) {
        logger.info(String.valueOf(e));
    }

    logger.debug(jsonInString);

    return jsonInString;
}