如何检索在谷歌应用引擎端点中创建的自动生成的实体密钥？

Question

我们正在使用 GAE 实现一个 android 项目，我们在 AppEngine 项目中创建了一个实体，我们希望在客户端使用它。我们需要检索刚刚创建的实体的键。

实体代码：

@Entity
public class Poll {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Key keyPoll;
private String title;
private String creator;
private Date creationDate;
private String close;

public Key getKeyPoll() {
    return keyPoll;
}

public String getTitle() {
    return title;
}

public String getCreator() {
    return creator;
}

public Date getCreationDate() {
    return creationDate;
}

public String getClose() {
    return close;
}

public void setTitle(String title) {
    this.title = title;
}

public void setCreator(String creator) {
    this.creator = creator;
}

public void setCreationDate(Date date) {
    creationDate = date;
}
public void setClose(String state) {
    close = state;
}
}

客户端代码：

  private class PollTask extends AsyncTask<Void, Void, Void> {

        @Override
        protected Void doInBackground(Void... params) {

                  //Entity creation
          Poll poll = new Poll();


                  poll.setCreator("Bill");
          poll.setCreationDate(new DateTime(System.currentTimeMillis()));
          poll.setTitle(title);
          poll.setClose("n");

          Pollendpoint.Builder builder = new Pollendpoint.Builder(
              AndroidHttp.newCompatibleTransport(), new JacksonFactory(),
              null);

          builder = CloudEndpointUtils.updateBuilder(builder);

          Pollendpoint endpoint = builder.build();


          try {
            endpoint.insertPoll(poll).execute();
          } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
          }

                  //this is another entity that store in its field "pollK" the key of
                  // the previous entity "poll"
          PollImg pollImg = new PollImg();


          pollImg.setPollK(poll.getKeyPoll());
          pollImg.setImageK(imageKey);

          Pollimgendpoint.Builder imgBuilder = new Pollimgendpoint.Builder(
              AndroidHttp.newCompatibleTransport(), new JacksonFactory(),
              null);

          imgBuilder = CloudEndpointUtils.updateBuilder(imgBuilder);

          Pollimgendpoint imgEndpoint = imgBuilder.build();


          try {
            imgEndpoint.insertPollImg(pollImg).execute();
          } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
          }


          return null;
        }
      }

问题是 poll.getKeyPoll() 会返回一个空值，即使实体“poll”已正确创建并在服务器上可见。

insertPoll 方法代码：

/**
 * This inserts a new entity into App Engine datastore. If the entity already
 * exists in the datastore, an exception is thrown.
 * It uses HTTP POST method.
 *
 * @param poll the entity to be inserted.
 * @return The inserted entity.
 */
@ApiMethod(name = "insertPoll")
public Poll insertPoll(Poll poll) {
    EntityManager mgr = getEntityManager();
    try {

        mgr.persist(poll);
    } finally {
        mgr.close();
    }
    return poll;
}

Answer 1

更正答案：根据您显示的代码，对象poll 是您首先创建的本地对象，然后通过网络将其发送到 App Engine 后端。您的 insertPoll 方法看起来不错。

但是，您不能期望代码会自动填充您的 poll 对象的 Key 字段，因为该对象仍然是本地的，仅此而已。要检索实际值，您需要查看代码中 endpoint.insertPoll(poll).execute(); 的响应值。这将返回 Poll 对象，然后您可以查询该对象以获取 Key 值。