Find documents matching ObjectIDs in a foreign array - mongodb

Question

I have a collection Users:
{
_id: "5cds8f8rfdshfd"
name: "Ted"
attending: [ObjectId("2cd9fjdkfsld")]
}
I have another collection Events:
{
_id: "2cd9fjdkfsld"
title: "Some Event Attended"
},
{
_id: "34dshfj29jg"
title: "Some Event NOT Attended"
}
I would like to return a list of all events being attended by a given user. However, I need to do this query from the Events collection as this is part of a larger query.
I have gone through the following questions:
$lookup on ObjectId's in an array - This question has the array as a local field; mine is foreign
MongoDB lookup when foreign field is an array of objects - The array is of objects themselves
MongoDB lookup when foreign field is an array
I have tried various ways of modifying the above answers to fit my situation but have been unsuccessful. The second answer from the third question gets me closest but I would like to filter out unmatching results rather than have them returned with a value of 0.
My desired output:
[
{
_id: "2cd9fjdkfsld"
title: "Some Event Attended"
},
]

Answer 1

One option would be like this:
db.getCollection('Events').aggregate({
$lookup: // join
{
from: "Users", // on Users collection
let: { eId: "$_id" }, // keep a local variable "eId" that points to the currently looked at event's "_id"
pipeline: [{
$match: { // filter where
"_id": ObjectId("5c6efc937ef75175b2b8e7a4"), // a specific user
$expr: { $in: [ "$$eId", "$attending" ] } // attends the event we're looking at
}
}],
as: "users" // push all matched users into the "users" array
}
}, {
$match: { // remove events that the user does not attend
"users": { $ne: [] }
}
})
You could obviously get rid of the users field by adding another projection if needed.