在googleappengine(GAE)上,如何搜索Key/ID字段?

我有以下代码(Java,GAE):

// Much earlier:
playerKey = KeyFactory.keyToString(somePlayer.key);

// Then, later...
PersistenceManager pm = assassin.PMF.get().getPersistenceManager();

Key targetKey = KeyFactory.stringToKey(playerKey);
Query query = pm.newQuery(Player.class);
query.setFilter("__key__ == keyParam");
query.declareParameters("com.google.appengine.api.datastore.Key keyParam");
List<Player> players = (List<Player>) query.execute(targetKey); // <-- line 200

从而产生此错误:

javax.jdo.JDOFatalUserException: Unexpected expression type while parsing query.  Are you certain that a field named __key__ exists on your object?
    at org.datanucleus.jdo.NucleusJDOHelper.getJDOExceptionForNucleusException(NucleusJDOHelper.java:354)
    at org.datanucleus.jdo.JDOQuery.execute(JDOQuery.java:252)
    at myapp.Player.validPlayerWithKey(Player.java:200)
    // [etc., snip]

但我不知道它想要什么。我试着搜索jdoid字段,我以为我读到的是一个特殊的名字 __key__ , in the documentation

我两个都试过了

query.setFilter("__key__ == keyParam");

query.setFilter("ID == keyParam");

同样的结果。那么,我做错了什么?或者,更重要的是,我该如何正确地做到这一点?

编辑:

Player result = null;
if (playerKey == null)
{
    log.log(Level.WARNING, "Tried to find player with null key.");
}
else
{
    PersistenceManager pm = assassin.PMF.get().getPersistenceManager();

    try {
        result = (Player) pm.getObjectById(Player.class, playerKey);
    } catch (javax.jdo.JDOObjectNotFoundException notFound) {
        // Player not found; we will return null.
        result = null;
    }

    pm.close();
}

return result;