javascript - MongoClient GetData 例程的 NodeJS ASync 调用

Question

下面的代码混合了https://www.w3schools.com/nodejs/nodejs_mongodb_find.asp 和 https://stackoverflow.com/questions/49982058/how-to-call-an-async-function#:~: text=Putting%20the%20async%20keyword%20before,a%20promise%20to%20be%20resolved。

当您查看代码下方的 console.log 时，事情似乎出了问题。我想通过使函数异步并使用 .then 来避免这些问题。

我希望 MongoDB 数据检索功能与 app.get 功能分开。没有数据返回到 get 请求。我猜 app.get 代码在函数返回值之前已经失败并结束。我需要纠正什么？

async function getLanguageTranslationData(fromLanguage, toLanguage) {
    console.log("Started getLanguageTranslationData")
    const databaseUrl = "mongodb://localhost:27017"
    const databaseName = 'MyCompanyPOC'
    
    mongoClient.connect(databaseUrl, function(err, conn) {
        if (err) throw err; 
        const collectionName = "Phrases";
        var dbo = conn.db(databaseName)
        var query = 
                { $and: 
                       [ {masterLanguage: fromLanguage},
                         {localizedLanguage: toLanguage} 
                       ]
                }
        console.log("query=" + JSON.stringify(query)); 
        console.log("about to retrieve data"); 
        dbo.collection(collectionName).find(query).toArray( 
             function(err, result) {
                  if (err) throw err; 
                  console.log("Back from mongoDB.find()")
                  console.log(JSON.stringify(result))
                  return result 
                  conn.close()
           }) 
    })  
}


app.get("/api/gettranslations/:fromLanguage/:toLanguage", 
        async function(req, res) {
  console.log("Backend: /api/gettranslations method started: " + 
     " fromLanguage=" + req.params.fromLanguage + 
     " toLanguage=" + req.params.toLanguage)
  
  getLanguageTranslationData(
                             req.params.fromLanguage, 
                             req.params.toLanguage)
        .then((arrayResult) => { 
           console.log("got arrayResult back from getLanguageTranslationData")
           res.status(200).json(arrayResult)
           console.log("Backend: End of /api/gettranslations process")
         })
}) 

Node.JS 控制台输出：

listening on port 3001
Backend: /api/gettranslations method started:  fromLanguage=en-US toLanguage=es-MX
Started getLanguageTranslationData
(node:44572) DeprecationWarning: current Server Discovery and Monitoring engine is deprecated, and will be removed in a future version. To use the new Server Discover and Monitoring engine, pass option { useUnifiedTopology: true } to the MongoClient constructor.
got arrayResult back from getLanguageTranslationData
Backend: End of /api/gettranslations process
query={"$and":[{"masterLanguage":"en-US"},{"localizedLanguage":"es-MX"}]}
about to retrieve data
Back from mongoDB.find()
[{"_id":"5f403f7e5036d7bdb0adcd09","masterLanguage":"en-US","masterPhrase":"Customers","localizedLanguage":"es-MX","localizedPhrase":"Clientes"},{ etc... 

标签： javascriptnode.jsasync-await

Answer 1

问题是getLanguageTranslationData应该返回 apromise以便您可以将其用作处理程序中的承诺，但在您的情况下，调用getLanguageTranslationData将返回 undefined，因为由于 nodejs的性质，此函数内的所有代码都将异步执行。non-blocking

所以你可以做的是getLanguageTranslationData像这样从你的函数中返回承诺。

function getLanguageTranslationData(fromLanguage, toLanguage) {
    const databaseUrl = "mongodb://localhost:27017"
    const databaseName = 'MyCompanyPOC'
    
    return new Promise((resolve, reject)=>{
        mongoClient.connect(databaseUrl, function(err, conn) {
            if (err) reject(err); 
            else{
                const collectionName = "Phrases";
                var dbo = conn.db(databaseName)
                var query = 
                        { $and: 
                               [ {masterLanguage: fromLanguage},
                                 {localizedLanguage: toLanguage} 
                               ]
                        }
                dbo.collection(collectionName).find(query).toArray( 
                     function(err, result) {
                          if (err) reject(err); 
                          else
                          resolve(result);    
                   }) 
            }
           
        }) 
    }) 
}

然后在你的处理程序中使用 await 来使用返回的承诺

app.get("/api/gettranslations/:fromLanguage/:toLanguage", 
  async function(req, res) {
      try{
        let arrayResult = await getLanguageTranslationData(req.params.fromLanguage, req.params.toLanguage);
        res.status(200).json(arrayResult)
      }catch(err){
        // return error
      }
}) 

上面的代码将为您提供您需要做什么的要点，实际代码可能会根据您的需要而有所不同。

您可以从这里参考async-await