Return an array element of an aggregation in an MongoDB Atlas (4.2) trigger function - mongodb

Question

So I am currently testing with returning an array element of a an aggregation in an MongoDB Atlas (4.2) trigger function:
exports = function(changeEvent) {
const collection = context.services.get(<clusterName>).db(<dbName>).collection(<collectionName>);
var aggArr = collection.aggregate([
{
$match: { "docType": "record" }
},
..,
{
$group: {
"_id": null,
"avgPrice": {
$avg: "$myAvgPriceFd"
}
}
}
]);
return aggArr;
};
Which outputs:
> result:
[
{
"_id": null,
"avgPrice": {
"$numberDouble": "18.08770081782988165"
}
}
]
> result (JavaScript):
EJSON.parse('[{"_id":null,"avgPrice":{"$numberDouble":"18.08770081782988165"}}]')
As you can see this is returned as one object in an array (I then intend to use the avgPrice value to update a field in a document in the same collection). I have tried to extract the object from the array with aggArr[0] or aggArr(0) - both resulting in:
> result:
{
"$undefined": true
}
> result (JavaScript):
EJSON.parse('{"$undefined":true}')
or by using aggArr[0].avgPrice as per this solution which fails with:
> error:
TypeError: Cannot access member 'avgPrice' of undefined
> trace:
TypeError: Cannot access member 'avgPrice' of undefined
at exports (function.js:81:10(163))
at function_wrapper.js:5:30(18)
at <eval>:13:8(6)
at <eval>:2:15(6)
Any pointers are most welcome because this one has me stumped for now!

Answer 1

I had the same problem, and figured it out. You have to append the .toArray() function to the aggregation call, where you have.
collection.aggregate(pipeline_steps).toArray()
Here's an example:
const user_collection = context.services
.get("mongodb-atlas")
.db("Development")
.collection("users");
const search_params = [
{
"$search": {
"index": 'search_users',
"text": {
"query": value,
"path": [
"email", "first_name", "last_name"
],
"fuzzy":{
"prefixLength": 1,
"maxEdits": 2
}
}
}
}
];
const search_results = await user_collection.aggregate(search_params).toArray();
const results = search_results
return results[0]
Here's the documentation showing how to convert the aggregation to an array.