SoftInstigate/restheart

@RegisterPlugin(name = "csvTransformer",
    interceptPoint = InterceptPoint.RESPONSE,
    description = "transform the response to CSV format",
    enabledByDefault = true)
public class CsvTransformer implements MongoInterceptor {
    @Override
    public void handle(MongoRequest request, MongoResponse response) {
        var docs = response.getContent().asArray();
        var sb = new StringBuilder();

        // add the header
        if (docs.size() > 0) {
            docs.get(0).asDocument().keySet().forEach(k -> sb.append(k).append(","));
            sb.append("\n");
        }

        // add rows
        docs.stream()
            .map(BsonValue::asDocument)
            .forEach(fdoc -> {
                sb.append(fdoc.entrySet().stream()
                    .map(e -> e.getValue())
                    .map(v -> BsonUtils.toJson(v))
                    .collect(Collectors.joining(",")));

                sb.append("\n");
            });

        response.setContentType("text/csv");

        response.setCustomSender(() ->  response.getExchange().getResponseSender().send(sb.toString()));
    }

    @Override
    public boolean resolve(MongoRequest request, MongoResponse response) {
        return request.isGet()
            && request.isCollection()
            && response.getContent() != null
            && request.getQueryParameterOfDefault("csv", null) != null;
    }
}