@Override
  public Object convertToTableKeyFormat(SamzaSqlRelRecord relRecord) {
    if (relRecord.getFieldValues().get(0) instanceof SamzaSqlRelRecord) {
      relRecord = (SamzaSqlRelRecord) relRecord.getFieldValues().get(0);
    }
    return relRecord.getFieldValues()
        .stream()
        .map(x -> x == null ? null : x.toString())
        .collect(Collectors.toList())
        .get(0);
  }
A sample {@link SamzaRelTableKeyConverter} used in tests to convert the join key to table format.
samza-sql/src/test/java/org/apache/samza/sql/util/SampleRelTableKeyConverter.java