I have a MongoDB database. I'm using Javascript and Node.js (using Mangoose) to access this database. I need to return an array with the name of all the collections in the database. So I use a code that follows.
let connection = mongoose.createConnection(process.env.MONGODB_URI + "/" + dbname);
// Wait 10 seconds for Mongoose to establish the connection.
await new Promise(r => setTimeout(r, 10000));
return connection.db.collections()
.then(stations=>stations.map(stations=>stations.collectionName))
.catch(reason => {
console.error("Error : "+reason);
return null;
});
The code above is working correctly. Now, I'm trying to do the same process asynchronously. For this, I am using the following code.
async function station_list_all(dbname){
return new Promise((resolve,reject) => {
try {
let connection = mongoose.createConnection(process.env.MONGODB_URI + "/" + dbname);
resolve(connection);
}catch(err){
reject(new Error("DB not found!"));
}
})
.then(connection => connection.db)
.then(db => db.collections())
.then(stations=>stations.map(station=>station.collectionName))
.catch(err => {
console.error("Error : "+err);
return null;
});
}
Unfortunately, instead of the collection names, I get the message: Error : TypeError: Cannot read property 'collections' of undefined.
I think db is returning undefined, but... Shouldn't the code wait until db has a valid value?
Try with async await:
try {
await mongoose.connect('mongo url with db name');
// other process
} catch (error) {
handleError(error);
}
Or you can connect using callback:
try {
mongoose.connect('mongo url with db name').then(()=>{
// other process
});
} catch (error) {
handleError(error);
}
In Same way you can try with promise also. There is no need use settimeout.
For More Ref Please visit: Mongoose Connections