code: public static String joinParentField(Settings settings) {
          if (StringUtils.hasText(settings.getMappingJoin())) {
              return settings.getMappingJoin().concat(".parent");
          }
          return null;
      }
docs: If "es.mapping.join" is set, this returns the field name for the join field's parent sub-field.
  @param settings to pull info from
  @return the parent sub-field to pull routing information from.