code: @InterfaceAudience.Public
      public String getDocumentRevisionId() {
          // Get the revision id from either the embedded document contents,
          // or the '_rev' or 'rev' value key:
          String rev = null;
          if (documentRevision != null)
              rev = documentRevision.getRevID();
          if (rev == null) {
              if (value instanceof Map) {
                  Map<String, Object> mapValue = (Map<String, Object>) value;
                  rev = (String) mapValue.get("_rev");
                  if (rev == null) {
                      rev = (String) mapValue.get("rev");
                  }
              }
          }
          return rev;
      }
docs: The revision ID of the document this row was mapped from.