Google App Engine Keys Only Query with CURSOR答案

【问题标题】：Google App Engine Keys Only Query with CURSORGoogle App Engine Keys Only Query with CURSOR
【发布时间】：2018-06-15 10:35:28
【问题描述】：

我正在使用具有 10000 多个实体的 PersistenceManager 查询获取数据存储实体，我只想获取实体的键，因为我使用限制为 1000 的游标，但获取后游标为空

PersistenceManager       pm     =   PMF.get().getPersistenceManager();
      Query                       q     = null;
      List<String>           idList     = null;
      int               cursorLimit     =   1000;
      Map<String, Object> cursorMap     =   null;
      Cursor              cursorObj     =   null;
      CommonUtil            utility     =   new CommonUtil();
      HashMap<String, Object> responseMap = null;



        responseMap = new HashMap<String, Object>();
        Query   q   = pm.newQuery("select id from " + Person.class.getName());

        if( requestMap.containsKey("cursor") )
        {
            cursorMap     = new HashMap<String,Object>();
            cursorObj     = Cursor.fromWebSafeString( String.valueOf( requestMap.get("cursor") )  );
            cursorMap.put(JDOCursorHelper.CURSOR_EXTENSION, cursorObj);
            q.setExtensions(cursorMap);
        }
        q.setRange(0, cursorLimit);
        idList = (List<String>) q.execute();
        cursorObj       = JDOCursorHelper.getCursor(idList);

        responseMap.put("contactIdList", idList);

        if(idList.size() == cursorLimit && cursorObj != null)
            responseMap.put("cursor", cursorObj.toWebSafeString());
        else
            responseMap.put("cursor", "");

        q.closeAll();
        pm.close();

但是 cursorObj 总是为空，有没有人提出这个问题或如何克服它

光标在获取整个实体时工作正常，但在键中仅查询它不能按预期工作

【问题讨论】：

标签： google-app-engine google-cloud-datastore datastore

【解决方案1】：

试试keys-only query：

Query<Key> query = Query.newKeyQueryBuilder().setKind("Task").build();

【讨论】：

【解决方案2】：

我当天用下面的代码解决了上面的光标问题，谢谢

final DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
          FetchOptions fetchOptions = FetchOptions.Builder.withLimit(cursorLimit);

          if( requestMap.containsKey("cursor"))
          {
              fetchOptions.startCursor(Cursor.fromWebSafeString(requestMap.get("cursor") ));
          }

            q = new com.google.appengine.api.datastore.Query("Contact").setKeysOnly();              
            PreparedQuery pq = datastore.prepare(q);                                
            results = pq.asQueryResultList(fetchOptions);               
            cursorObj = results.getCursor();

这个 LowLevel API 解决了问题

【讨论】：