javascript - TypeError：尝试使用 Mongodb Atlas Online 时无法读取未定义的属性“db”

Question

[nodemon] 启动node server.js C:\Users\Abhay\Desktop\todo-app\node_modules\mongodb\lib\utils.js:725 抛出错误；^

类型错误：无法读取 C:\Users\Abhay\Desktop\todo-app\server.js:8:17 在 C:\Users\Abhay\Desktop\todo-app\node_modules\mongodb\lib 处未定义的属性“db” \utils.js:722:9 在 C:\Users\Abhay\Desktop\todo-app\node_modules\mongodb\lib\mongo_client.js:223:23 在 C:\Users\Abhay\Desktop\todo-app\node_modules \mongodb\lib\operations\connect.js:279:21 在 QueryReqWrap.callback (C:\Users\Abhay\Desktop\todo-app\node_modules\mongodb\lib\core\uri_parser.js:56:21) 在 QueryReqWrap .onresolve [as oncomplete] (dns.js:202:10) [nodemon] 应用程序崩溃 - 在开始之前等待文件更改...

let express = require('express')
let mongodb = require('mongodb')
let app = express()
let db

let connectionString = 'mongodb+srv://todoAppUser:[e-mail]/TodoApp?retryWrites=true&w=majority'
mongodb.connect(connectionString, {useNewUrlParser: true, useUnifiedTopology: true}, function(err, client) {
    db = client.db()
    app.listen(3000)
})

标签： javascriptnode.jsmongodbcrudmongodb-atlas

Answer 1

您似乎正在尝试使用MongoClient的静态连接方法来连接到您的数据库，但您没有使用 MongoClient 类本身。

要连接到任何数据库，您将需要一个连接的 MongoClient 实例。使用静态连接方法，可以通过以下方式实现：

const mongodb = require("mongodb");


const connectionURL = "mongodb+srv://your-connection-srv-here"
const dbName = "your_db_name"

//get MongoClient
const MongoClient = mongodb.MongoClient;

let db = null;

MongoClient.connect(connectionURL,{
    useNewUrlParser: true,
    useUnifiedTopology: true
},(err,connectedClient) => {
    if(err){
        throw err;
    }
    //connectedClient will be the connected instance of MongoClient
    db = connectedClient.db(dbName);
    //now you can write queries

    db.collection("your_collection").find({}).toArray()
    .then(r => {
        console.log(r);
    }).catch(e => {
        console.error(`ERROR:`,e);
    })

})

但是，使用回调会很麻烦。根据上面链接的文档，如果未传递回调函数，Node.js 的 MongoDb 驱动程序中的大多数函数都会返回一个 Promise ，这非常方便。使用它，您可以编写一个函数，该函数返回一个将连接的实例解析到您的数据库的承诺。

const MongoClient = require('mongodb').MongoClient;


/*
    we draw the connection srv and the db name from the config to return just one instance of that db.
    Now this function call be called wherever a connection is needed
*/
const getDbInstance = (config) => new Promise((resolve,reject) => {
    const client = new MongoClient(config.dbUrl, {
        useNewUrlParser: true,
        useUnifiedTopology: true
    });
    client.connect((error) => {
        if(error){
            console.error(error);
            reject(error);
        }
        let db = client.db(config.dbName);
        resolve(db);
    })
})


const doSomeDbOperations = async() => {
    //hardcoding it here, but this config will probably come from environment variables in your project
    const config = {
        dbUrl: "mongodb+srv://your-connection-srv-here",
        dbName: "your_db_name"
    };

    try{
        const db = await getDbInstance(config);

        //do whatever querying you wish here

    }catch(e){
        console.error(`ERROR: `,e);
    }

}

doSomeDbOperations();