代码之家 › 专栏 › 技术社区 › Carl

googleappengine:从任务中的数据存储中获取对象时出现问题

task google-cloud-datastore queue google-app-engine java

Carl · 技术社区 · 14 年前

我的应用程序构造了一个 Parent Child ren,然后启动任务在任,就像这样:

public static Parent make(User owner, List<Integer> data, int size) {
        Parent result = new Parent(owner,data,size);
        PersistenceManager pm = PersistenceSource.get();
        Transaction tx = pm.currentTransaction();

        try {
            tx.begin();
            result = pm.makePersistent(result);
            for (int i=0; i<size; pm.makePersistent(new Child(result,i++)));
            pm.close();
            tx.commit();
        } finally {
            if (tx.isActive()) { tx.rollback(); result=null; }
        }

        if (result!=null) {
            Queue q = QueueFactory.getDefaultQueue();
            for (Child c : result.getChild()) {
                q.add(url("/task/child").param("key", KeyFactory.keyToString(c.getKey())).method(Method.PUT));
            }
        }
        pm.close();
        return result;
    }

public void doPut(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
    PersistenceManager pm = PersistenceSource.get();

    Child c = pm.getObjectById(Child.class, KeyFactory.stringToKey(request.getParameter("key"))); //...

它在寻找目标时死去:

Could not retrieve entity of kind Child with key Child(24)
org.datanucleus.exceptions.NucleusObjectNotFoundException: Could not retrieve entity of kind Child with key Child(24)

有什么见解吗?另外,如果有关系的话,父-子关系由父定义为子中的一个字段(因此将父定义为arg)。

2 回复 | 直到 14 年前

Carl 14 年前

在进行了一些插入操作之后,下面的步骤将能够正确地检索所需的 Child

    Key k = new KeyFactory
        .Builder(Parent.class.getSimpleName(), Long.valueOf(request.getParameter("parent")))
        .addChild(Child.class.getSimpleName(), Long.valueOf(request.getParameter("child")))
        .getKey();
    Child c = pm.getObjectById(Child.class, k);

对于为什么type+id不足以获取我想要的东西,来自非数据存储世界的我仍然有点迷惑。这似乎相当于在sqlland中知道table+主键,文档似乎表明该键包含所有父信息,因此拥有它就足以直接执行一个操作 pm.getObjectId(Child, KeyFactory./* etc */)

Donato Szilagyi 11 年前

public Object retrieveChildByKey(Key parent, Class childClass, int idChild){
    try{
        pm = PMF.get().getPersistenceManager();
        Key objectKey = KeyFactory.createKey(parent, childClass.getSimpleName(), idChild);
        Object result = pm.getObjectById(childClass, objectKey);
        return result;
    }
    catch (DatastoreTimeoutException ex) {
        // Display a timeout-specific error page
        message = "Timeout: An error has occurred: Was not possible to retrieve the Object";
        System.out.println(message);
        ex.printStackTrace();
    }
    catch(Exception ex){
        message = "Timeout: An error has occurred: Was not possible to retrieve the Object";
        System.out.println(message);
        ex.printStackTrace();
    }finally{
        pm.close();
    }
    return null;
}