code: public org.apache.lucene.search.SortField sortField(Schema schema) {
          if (field == null || field.trim().isEmpty()) {
              throw new IllegalArgumentException("Field name required");
          }
          ColumnMapper columnMapper = schema.getMapper(field);
          if (columnMapper == null) {
              throw new IllegalArgumentException("No mapper found for sortFields field " + field);
          } else {
              return columnMapper.sortField(field, reverse);
          }
      }
docs: Returns the Lucene {@link org.apache.lucene.search.SortField} representing this {@link SortField}.
  
  @param schema The {@link Schema} to be used.
  @return the Lucene {@link org.apache.lucene.search.SortField} representing this {@link SortField}.