private String fetchBatches(
      DBCollection collection, Set<String> loadedUris, String lastDCIdFetched) {

    BulkWriteOperation builder = collection.initializeUnorderedBulkOperation();
    String prevDcId = lastDCIdFetched;

    DCQueryParameters params;
    params =
        lastDCIdFetched == null
            ? new DCQueryParameters(searchCronField, DCOrdering.DESCENDING)
            : new DCQueryParameters(
                searchCronField,
                DCOrdering.DESCENDING,
                DCOperator.LT,
                "\"" + lastDCIdFetched + "\"");
    // (LT & descending order to leave possible null geo:name values at the end rather than having
    // to skip them)

    logger.debug("Querying the Data Core");
    long queryStart = System.currentTimeMillis();
    List<DCResource> resources = datacore.findResources(project, areaModel, params, 0, batchSize);
    long queryEnd = System.currentTimeMillis();
    logger.debug("Fetched {} resources in {} ms", resources.size(), queryEnd - queryStart);

    boolean hasOne = false;

    for (DCResource res : resources) {
      String name = null;

      for (Languages language : Languages.values()) {

        GeographicalArea area = geographicalDAO.toGeographicalArea(res, language.getLanguage());
        if (area == null) {
          continue;
        }

        if (!loadedUris.contains(language.getLanguage() + area.getUri())) {
          hasOne = true;
          if (name == null) {
            name = area.getName();
            // logger.debug("{} - {}", name, area.getUri());
          }

          DBObject dbObject = new BasicDBObject();
          mappingMongoConverter.write(area, dbObject);

          builder.insert(dbObject);

          loadedUris.add(language.getLanguage() + area.getUri());
        } else {
          if (name == null) {
            name = area.getName();
            logger.debug(
                "Area {} already inserted for language {}, skipping",
                area.getName(),
                language.getLanguage());
          }
        }
      }

      String id =
          res
              .getUri(); //  ID resource in DC is always encoded, so to match values we need to
                         // encoded as well
      if (id != null) {
        lastDCIdFetched = id;
      }
    }

    if (hasOne) {
      long st = System.currentTimeMillis();
      builder.execute();
      long durationSave = System.currentTimeMillis() - st;
      logger.debug(
          "Saved resources; total save time={} ms (avg = {} ms)",
          durationSave,
          durationSave / resources.size());
    }

    if ((prevDcId != null && prevDcId.equals(lastDCIdFetched)) || resources.size() < batchSize) {
      return null;
    } else return lastDCIdFetched;
  }