code: public static Map<Identity, JsonNode> getDocumentIdMap(List<JsonNode> list, List<String> identityFields) {
          Map<Identity, JsonNode> map = new HashMap<>();
          if (list != null) {
              LOGGER.debug("Getting doc IDs for {} docs, fields={}", list.size(), identityFields);
              for (JsonNode node : list) {
                  Identity id = new Identity(node, identityFields);
                  LOGGER.debug("ID={}", id);
                  map.put(id, node);
              }
          }
          return map;
      }
docs: Build an id-doc map from a list of docs