private Fields addGeneratedTermVectors(
      Engine.GetResult get,
      Fields termVectorsByField,
      TermVectorRequest request,
      Set<String> selectedFields)
      throws IOException {
    /* only keep valid fields */
    Set<String> validFields = new HashSet<>();
    for (String field : selectedFields) {
      FieldMapper fieldMapper = indexShard.mapperService().smartNameFieldMapper(field);
      if (!isValidField(fieldMapper)) {
        continue;
      }
      // already retrieved, only if the analyzer hasn't been overridden at the field
      if (fieldMapper.fieldType().storeTermVectors()
          && (request.perFieldAnalyzer() == null
              || !request.perFieldAnalyzer().containsKey(field))) {
        continue;
      }
      validFields.add(field);
    }

    if (validFields.isEmpty()) {
      return termVectorsByField;
    }

    /* generate term vectors from fetched document fields */
    GetResult getResult =
        indexShard
            .getService()
            .get(
                get,
                request.id(),
                request.type(),
                validFields.toArray(Strings.EMPTY_ARRAY),
                null,
                false);
    Fields generatedTermVectors =
        generateTermVectors(
            getResult.getFields().values(), request.offsets(), request.perFieldAnalyzer());

    /* merge with existing Fields */
    if (termVectorsByField == null) {
      return generatedTermVectors;
    } else {
      return mergeFields(termVectorsByField, generatedTermVectors);
    }
  }