code: public static Bson geoHaystack(final String fieldName, final Bson additional) {
          notNull("fieldName", fieldName);
          return compoundIndex(new BsonDocument(fieldName, new BsonString("geoHaystack")), additional);
      }
docs: Create an index key for a geohaystack index on the given field.
  
  <p>
  <strong>Note: </strong>For queries that use spherical geometry, a 2dsphere index is a better option than a haystack index.
  2dsphere indexes allow field reordering; geoHaystack indexes require the first field to be the location field. Also, geoHaystack
  indexes are only usable via commands and so always return all results at once..
  </p>
  
  @param fieldName the field to create a geoHaystack index on
  @param additional the additional field that forms the geoHaystack index key
  @return the index specification
  @mongodb.driver.manual core/geohaystack geoHaystack index