Execute an aggregation as a promise in NodeJS

[Newbie (Mongodb Newbie)]

How to execute an aggregation from NodeJS that outputs “$out” the results to a collection as the last stage of the aggregation? The aggregation works if I call it from the MongoDB Shell, but I would like to do the same in node.js without it returning the result to the client. I want to result to be sent to another collection in the same database and not to the client application.

[steevej (Steeve Juneau)]

The same aggregation should produce the same result.

Have you tried it?

How are the results differ?

[Newbie (Mongodb Newbie)]

I get “in_db.collection(…).aggregate(…).then is not a function” when I try running the sample code below.

async function aggregateBrands(in_db, out_db) {
in_db.collection(‘brands’).aggregate(
[{
“$group”:
{“name”: “$brand_name”,
“brand_id”: {"$addToSet": “$brand_id”},
“count”: {"$sum": 1}
}
},
{ $out: {db:“out_db”, coll:“brand”}}
]).then(result => {
console.log(result)
}).catch(err => {
console.log(err.message)
});
}

[steevej (Steeve Juneau)]

If you look at the documentation at Collection | mongodb, you will see that aggregate does not return a Promise but an AggregationCursor.

This means you cannot use then(), hence the error then is not a function. You will need to use one of the method that returns a Promise. In this case, since no documents is output after $out, I am not too sure which one would be a good fit. I would start with tryNext().