Question

Tracker.autorun(function() {
  DATA.find().observeChanges({
    added: function(id, doc) {
       console.log(doc);
    }
  });
});

在服务器上调用此代码。每次meteor服务器启动时，added函数都会触发数据库中的每个项目。有没有办法让added回调仅在添加新项目时触发？

Answer 1

首次运行added时，将为结果集中的每个文档调用

observeChanges。诀窍是在初始化期间忽略回调。我在回答this问题时有一个扩展示例，但此代码应该适合您：

(function() {
  var initializing = true;
  DATA.find().observeChanges({
    added: function(id, doc) {
      if (!initializing) {
        console.log(doc);
      }
    }
  });
  initializing = false;
})();

请注意，Tracker.autorun是仅限客户端的功能。在服务器上，我认为它只执行一次。

Answer 2

我很长时间都在努力。出于某种原因，David的回答对我不起作用 - 它是在初始化变量设置为false后触发的。

Avi的这种模式对我来说很成功：

var usersLoaded = false;
Meteor.subscribe("profiles", function () {
    // at this point all new users sent down are legitimately new ones
    usersLoaded = true;
});

Meteor.users.find().observe({
    added: function(user) {
        if (usersLoaded) {
            console.log("New user created: ", user);
        }
    }
});

Answer 3

由于是初始化问题，您可以这样做。

public class TwitterStreaming {
     // setup kafka : 
     public static final String ZKQuorum = "localhost:2181";
     public static final String ConsumerGroupID = "ingi2145-analytics";
     public static final String ListTopics = "newTweet";
     public static final String ListBrokers = "localhost:9092"; // I'm not sure about ...

    @SuppressWarnings("deprecation")
public static void main(String[] args) throws Exception {
    // Location of the Spark directory
    String sparkHome = "usr/local/spark";
    // URL of the Spark cluster
    String sparkUrl = "local[4]";
    // Location of the required JAR files
    String jarFile = "target/analytics-1.0.jar";
// Generating spark's streaming context
JavaStreamingContext jssc = new JavaStreamingContext(
  sparkUrl, "Streaming", new Duration(1000), sparkHome, new String[]{jarFile});
// Start kafka stream
HashSet<String> topicsSet = new HashSet<String>(Arrays.asList(ListTopics.split(",")));
HashMap<String, String> kafkaParams = new HashMap<String, String>();
kafkaParams.put("metadata.broker.list", ListBrokers);

//JavaPairReceiverInputDStream<String, String> kafkaStream = KafkaUtils.createStream(ssc, ZKQuorum, ConsumerGroupID, mapPartitionsPerTopics);
// Create direct kafka stream with brokers and topics
JavaPairInputDStream<String, String> messages = KafkaUtils.createDirectStream(
    jssc,
    String.class,
    String.class,
    StringDecoder.class,
    StringDecoder.class,
    kafkaParams,
    topicsSet
);

// get the json file :
   JavaDStream<String> json = messages.map(
        new Function<Tuple2<String, String>, String>() {
            public String call(Tuple2<String, String> tuple2) {
              return tuple2._2();
            }
   });

实际上这更优雅。

Answer 4

为查询提供与旧项目不匹配的选择器。如果将mongo ObjectID用作_id，您可以查询_id大于最新项目的项目：

const latest = DATA.findOne({}, {sort: {_id: -1}})
DATA.find({_id: {$gt: latest._id}}).observeChanges({
  added: function() { ... }
})

或createdAt时间戳：

const currentTime = new Date()
DATA.find({createdAt: {$gt: currentTime}}).observeChanges({
  added: function() { ... }
})

Answer 5

以下是解决此问题的另一种方法：

Meteor.subscribe('messages', function() {
    var messages = Messages.find();
    var msgCount = messages.count();

    messages.observe({
        addedAt: function(doc, atIndex) {
            if(atIndex > (msgCount - 1)) console.log('added');
        }
    });
});

只应在现有金额交付后为已添加的文档触发。重要的是，onReady Meteor.subscribe回调msgCount，以便select 'C:\images\'|| substr('00453434',1,2) || '\' || substr('00453434',3,2) || '\' || substr('00453434',5,2) || '\' || substr('00453434',7,2) || '.IMG' as fullPath from dual更改为您的订阅...例如，您正在对您的订阅进行分页。

cursor.observe() documentation

Meteor观察更改添加了对所有项目的服务器触发的回调

5 个答案: