code: @InterfaceAudience.Public
      public String getDocumentId() {
          // Get the doc id from either the embedded document contents, or the '_id' value key.
          // Failing that, there's no document linking, so use the regular old _sourceDocID
          String docID = null;
          if (documentRevision != null)
              docID = documentRevision.getDocID();
          if (docID == null) {
              if (value != null) {
                  if (value instanceof Map) {
                      Map<String, Object> props = (Map<String, Object>) value;
                      docID = (String) props.get("_id");
                  }
              }
          }
          if (docID == null)
              docID = sourceDocID;
          return docID;
      }
docs: The ID of the document described by this view row.  This is not necessarily the same as the
  document that caused this row to be emitted; see the discussion of the .sourceDocumentID
  property for details.