code: private Optional<Object> getWriterPartitionColumnValue(GenericRecord record) {
      if (!this.partitionColumns.isPresent()) {
        return Optional.absent();
      }
  
      for (String partitionColumn : this.partitionColumns.get()) {
        Optional<Object> fieldValue = AvroUtils.getFieldValue(record, partitionColumn);
        if (fieldValue.isPresent()) {
          return fieldValue;
        }
      }
      return Optional.absent();
    }
docs: Retrieve the value of the partition column field specified by this.partitionColumns