private String getAggregationOperation(final AggregateOperation operation) {
        String aggregationOperation = null;
        switch (operation) {
        case GEO_NEAR:
            aggregationOperation = "$geoNear";
            break;
        case MATCH:
            aggregationOperation = "$match";
            break;
        case GROUP:
            aggregationOperation = "$group";
            break;
        case LIMIT:
            aggregationOperation = "$limit";
            break;
        case SKIP:
            aggregationOperation = "$skip";
            break;
        case PROJECT:
            aggregationOperation = "$project";
            break;
        case SORT:
            aggregationOperation = "$sort";
            break;
        case UNWIND:
            aggregationOperation = "$unwind";
            break;
        }
        return aggregationOperation;
    }