小编pop*_*opo的帖子

为了研究Android服务,我写了一个测试程序,在屏幕上有三个按钮"绑定服务","解绑服务"和"发送回显".单击时,他们使用bindService(),unbindService()以及Messenger与该服务进行通信.

这是服务代码:

public class MessengerService extends Service {

private final Messenger mMessenger = new Messenger(new TempHandler());
private class TempHandler extends Handler {
    @Override
    public void handleMessage(Message msg) {
        switch (msg.what) {
        case MSG_SAY_HELLO:
            Toast.makeText(getApplicationContext(), "Hi, there.", Toast.LENGTH_SHORT).show();
            break;

        case MSG_SAY_GOODBYE:
            Toast.makeText(getApplicationContext(), "See you next time.", Toast.LENGTH_SHORT).show();
            break;

        case MSG_ECHO:
            Toast.makeText(getApplicationContext(), "Received " + msg.arg1 + " from client.", Toast.LENGTH_SHORT).show();

            Messenger replyMessenger = msg.replyTo;
            Message replyMsg = Message.obtain(null, MSG_ECHO, msg.arg1, 0);
            try {
                replyMessenger.send(replyMsg);
            } …

在mongodb中，我具有如下数据结构：

{
    "data" : [ 
        {
            "car" : [ 
                {
                    "model" : "aaa",
                }, 
                {
                    "model" : "bbb",
                }, 
                {
                    "model" : "ccc",
                }
            ],
            "user" : {
                "id" : "123"
            }
        }
    ],
}

我想对汽车模型领域进行分组和计数。因此，首先我尝试了如下展开方法：

.aggregate([
  { $project: {"data.car.model":1, "data.user.id":1} },
  { $unwind: {
      path: "$data.car",
      preserveNullAndEmptyArrays: true
      } },
  { $group: {
      _id: "$data.car.model",
      count: { $sum: 1 }
      } }
])

但是结果是：

{
    "_id" : [ ["aaa", "bbb", "ccc"] ],
    "count" : 1.0
}

因为模型没有展开，所以这不是我想要的。我希望的结果是：

[{_id:"aaa",count:1}, {_id:"bbb",count:1}, …