MongoDB return nested array of objects minus a property within the object (also .then() doesn't work with aggregation) - node.js

Question

I'm very new to mongodb and am having difficulty getting to a solution for my use case. For example I have the following document:
{
_id : ObjectId('5rtgwr6gsrtbsr6hsfbsr6bdrfyb'),
uuid : 'something',
mainArray : [
{
id : 1,
title: 'A',
array: ['lots','off','stuff']
},
{
id : 2,
title: 'B',
array: ['even','more','stuff']
}
]
}
I'd like to have the following returned:
{
uuid : 'something',
mainArray : [
{
id : 1,
title: 'A'
},
{
id : 2,
title: 'B'
}
]
}
I've tried various combinations of using findOne() and aggregate() with $slice and $project. With findOne(), if it worked at all, the who document would be returned. I am unable to test whether attempts at aggregating work because .then((ret)=>{}) promises don't seem to work in node.js for me with it (no issues with findOne). Calling a function like so
return db.myCollection.aggregate([
{
$match: {
_id:ObjectId(mongo_id)
}
},
{
$project : {
mainArray: {
id:1,
title:1
}
}
}
],function(err,res){
console.log(res)
return res
})
logs the entire function and not the droids I'm looking for.

Answer 1

You're missing toArray() method to obtain the actual result set. Instead you're returning the aggregation cursor object. Try this.
return db.myCollection.aggregate([matchCode,projectCode]).toArray().then(
data => {
console.log(data);
return data;
},
error => { console.log(error)});
The documnetation on aggregation cursor for MongoDB NodeJS driver can
be found here
http://mongodb.github.io/node-mongodb-native/3.5/api/AggregationCursor.html#toArray

Answer 2

This is an alternative solution (to the solution mentioned in the comment by #v1shva)
Instead of using aggregation you can use projection option of .findOne() operation.
db.myCollection.findOne(matchCode, {
projection: { _id: false, 'mainArray.array': false } // or { _id: -1, 'mainArray.array': -1 }
})