javascript - Node.js | MongoDB 计数() : Getting Count Before and After Inserting Data

Question

我正在尝试获取数据库中的项目计数以确认数据插入是否成功。

1. 插入前获取计数
2. 插入
3. 插入后获取计数
4. Console.log 摘要

注意:我知道这可以使用一些简单的函数来实现:

dbName.equal(insertSize, result.insertedCount)

但是，我是 JavaScript 新手，我想我遇到了实现异步回调的需要，所以我想弄清楚这一点。

插入函数

var insertMany = function() {

    // get initial count
    var count1 = getDbCount();

    // Insert new 'data'
    MongoClient.connect(url, function(err, db) {
        var col = db.collection(collectionName);
        col.insert(data, {w:1}, function(err,result) {});
        db.close();
    });

    /** This needs to be implemented through next/callback
        after the insert operation  **/
    var count2 = getDbCount(); 

    /** These final console logs should be executed after all
        other operations are completed **/    
    console.log('[Count] Start: ' + count1 + ' | End:' +count2);
    console.log('[Insert] Expected: ' + data.length + ' | Actual: ' + (count2 - count1));

};

获取DB计数函数

var getDbCount = function() {
    MongoClient.connect(url, function(err, db) {
        if (err) console.log(err);

        var col = db.collection(collectionName);

        col.count({}, function(err, count) {
            if (err) console.log(err);
            db.close();
            console.log('docs count: ' + count);
            // This log works fine
        });

    });

    return count; // this is returning as undefined since this is
                  // executing before the count operation is completed
};

我收到错误，因为返回是在所需操作完成之前发生的。

感谢您的帮助。

<小时/>

[编辑]带有 Promise 的 getCount 函数

我已将 Promise 添加到 getCount 函数作为开始:

var getCount = function() {

    var dbCount = 0;

    var promise = new Promise(function(resolve, reject) {

        MongoClient.connect(url, function(err, db) {

            if (err) {
                console.log('Unable to connect to server', err);
            } else {
                console.log('Database connection established:' + dbName);
            }

            // Get the collection
            var col = db.collection(collectionName);

            col.count({}, function(err, count) {
                if (err) console.log(err);
                db.close();
                console.log('docs count: ' + count);
                resolve(null);
                dbCount = count;
            });

        });

    });

    promise.then(function() {
        return dbCount;
    });

};

console.log(getCount());

输出仍然是: 不明确的 已建立数据库连接:testdb 文档数量:500

那么then({return count})代码仍然在promise {db.count()}之前执行。在数据库操作完成之前，它返回未定义。

Answer 1

一般来说，您遇到的问题是您希望函数返回时该值已经存在。对于异步函数来说，情况通常并非如此。有大量关于异步处理的信息(不要与 Java 中的并行处理相混淆)。

我创建了一个适合您情况的示例:

https://jsfiddle.net/rh3gx76x/1/

var dummyCount = 5
getCount = function() {
    return new Promise(function(resolve, reject) {
      setTimeout(function() { // this would be your db call, counting your documents
          resolve(dummyCount); // dummy for number of documents found
      }, 100 * Math.random());
  });
};

insertMany = function() {
    return new Promise(function(resolve, reject) {
      setTimeout(function() { // this would be your db call, writing your documents
          dummyCount += 2;
          resolve();
      }, 100 * Math.random());
  });
};

runIt = function(callback) {
   var count1;
   getCount().then(function(count) {
        console.log("First callback with value ", count);
      count1 = count;
        insertMany().then(function() {
         getCount().then(function(count2){
            console.log("Second callback with value ", count2);
                callback(null, count1, count2);
         });
      })
   })
}

runIt(function(err, count1, count2) {
   console.log("count1: " + count1 + ", count2: " + count2);
});

最后一件事。您可能想查看“async”包。这对解决这些问题有很大帮助，提供了很多帮助函数和控制流内容。