问题
The code below is a mix of https://www.w3schools.com/nodejs/nodejs_mongodb_find.asp and https://stackoverflow.com/questions/49982058/how-to-call-an-async-function#:~:text=Putting%20the%20async%20keyword%20before,a%20promise%20to%20be%20resolved.
When you look at the console.log below the code, things seem to be running out of order. I thought by making the function async and using the .then I would avoid those issues.
I want the MongoDB data retrieval function separate from the app.get function. No data is being returned to the get request. I guess the app.get code is falling through and ending before the function returns the value. What do I need to correct?
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 =
{ $and:
[ {masterLanguage: fromLanguage},
{localizedLanguage: toLanguage}
]
}
console.log("query=" + JSON.stringify(query));
console.log("about to retrieve data");
dbo.collection(collectionName).find(query).toArray(
function(err, result) {
if (err) throw err;
console.log("Back from mongoDB.find()")
console.log(JSON.stringify(result))
return result
conn.close()
})
})
}
app.get("/api/gettranslations/:fromLanguage/:toLanguage",
async function(req, res) {
console.log("Backend: /api/gettranslations method started: " +
" fromLanguage=" + req.params.fromLanguage +
" toLanguage=" + req.params.toLanguage)
getLanguageTranslationData(
req.params.fromLanguage,
req.params.toLanguage)
.then((arrayResult) => {
console.log("got arrayResult back from getLanguageTranslationData")
res.status(200).json(arrayResult)
console.log("Backend: End of /api/gettranslations process")
})
})
Node.JS Console output:
listening on port 3001
Backend: /api/gettranslations method started: fromLanguage=en-US toLanguage=es-MX
Started getLanguageTranslationData
(node:44572) DeprecationWarning: current Server Discovery and Monitoring engine is deprecated, and will be removed in a future version. To use the new Server Discover and Monitoring engine, pass option { useUnifiedTopology: true } to the MongoClient constructor.
got arrayResult back from getLanguageTranslationData
Backend: End of /api/gettranslations process
query={"$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...
回答1:
The thing is getLanguageTranslationData should return a promise so that you can use it as a promise in your handler, but in your case, call to getLanguageTranslationData will return undefined as all the code inside this function will execute asynchronously due to nodejs non-blocking nature.
So what you can do is return promise from your getLanguageTranslationData function like this.
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);
})
}
})
})
}
and then use await in your handler to use that promise returned
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
}
})
The above code will give you the gist of what you need to do, actual code may vary according to your needs.
You can refer async-await from here
回答2:
I got it working in this way, based on this example: Node.js, Mongo find and return data
@Namar's answer is probably correct too, but I was testing this the same time he posted. As the StackOverflow question/answer above notes, the up-to-date versions of MongoClient have support for promises. That post also shows how to put in a separate module, which is something I will probably do later this week.
function getLanguageTranslationData(fromLanguage, toLanguage) {
console.log("Started getLanguageTranslationData")
const databaseUrl = "mongodb://localhost:27017"
const databaseName = 'ShedCompanyPOC'
return mongoClient.connect(databaseUrl)
.then(function(conn) {
var collectionName = "UploadedDataeFromExcel";
var dbo = conn.db(databaseName)
var query =
{ $and:
[ {masterLanguage: fromLanguage},
{localizedLanguage: toLanguage}
]
}
console.log("query=" + JSON.stringify(query));
console.log("about to retrieve data");
var collection = dbo.collection(collectionName)
return collection.find(query).toArray();
}).then(function(result) {
console.log("Back from mongoDB.find()")
console.log(JSON.stringify(result))
//conn.close()
return result
});
}
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(function(arrayResult) {
console.log("got arrayResult back from getLanguageTranslationData")
res.status(200).json(arrayResult)
console.log("Backend: End of /api/gettranslations process")
}, function(err) {
console.log("The promise was rejected", err, err.stack)
})
})
来源：https://stackoverflow.com/questions/63605765/nodejs-async-call-of-mongoclient-getdata-routine