protected KV<Object, Object> convertToSamzaMessage(SamzaSqlRelMessage relMessage, Schema payloadSchema) {
    return new KV<>(relMessage.getKey(), convertToGenericRecord(relMessage.getSamzaSqlRelRecord(), payloadSchema));
  }
Convert the nested relational message to the output samza message.
samza-sql/src/main/java/org/apache/samza/sql/avro/AvroRelConverter.java