code: @Override
      public org.apache.lucene.search.SortField sortField(Schema schema) {
          if (field.equalsIgnoreCase("score")) {
              return FIELD_SCORE;
          }
          Mapper mapper = schema.mapper(field);
          if (mapper == null) {
              throw new IndexException("No mapper found for sortFields field '{}'", field);
          } else if (!mapper.docValues) {
              throw new IndexException("Field '{}' does not support sorting", field);
          } else {
              return mapper.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 equivalent Lucene sort field