code: @Override
      public void addUninvertingMappings(Map<String, Type> uninvertingMap) {
  
          for (String fieldName : getFieldNames()) {
              uninvertingMap.put(fieldName, Type.SORTED);
          }
  
      }
docs: To allow sorting on a field the field must be added to the map given to {@link org.apache.solr.uninverting.UninvertingReader#wrap(org.apache.lucene.index.DirectoryReader, Map)}.
  The method adds the configured fields.
  @param uninvertingMap the map to which the fields are added.