Mongoose aggregation with discriminator - node.js

Question

I use CosmosDB Mongo API with only one collection using discriminators.
I'm now facing a problem where I want to "join" other collections into one.
My common collection is named "alldata" and has discriminator keys (_type) "Session", "Room" and "User".
Session contains "roomId" and "speakerIds" (Array of user ids).
Session object:
{
"singleEntrance": false,
"accreditationRequired": false,
"closed": false,
"usersAtended": [],
"speakerIds": [
"5b573723930d9751768f266e",
"5b586b6032063a70632721d8"
],
"archived": false,
"_type": "Session",
"_id": "5b586f81ae744a7266524f84",
"title": "Bla bla",
"about": "",
"type": "SIS1",
"track": "Special Interest Session",
"venue": "Bla 1",
"roomId": "5b5731e764f0de4f9e4082ac",
"from": "2018-10-22T10:00:00.015Z",
"to": "2018-10-22T12:00:00.015Z"
}
Result I want:
{
"singleEntrance": false,
"accreditationRequired": false,
"closed": false,
"usersAtended": [],
"spikeri": [
{* LIST OF USER OBJECTS *}
],
"archived": false,
"_type": "Session",
"_id": "5b586f81ae744a7266524f84",
"title": "Bla bla",
"about": "",
"type": "SIS1",
"track": "Special Interest Session",
"venue": "Bla 1",
"room": {
"_id": "5b5731e764f0de4f9e4082ac",
"archived": false,
"_type": "Room",
"name": "Bla 1",
"location": "Hotel Bla",
"createdAt": "2018-07-24T14:04:23.370Z",
"updatedAt": "2018-07-24T14:04:23.370Z",
"__v": 0
},
"from": "2018-10-22T10:00:00.015Z",
"to": "2018-10-22T12:00:00.015Z"
}
Room works fine, but I just cannot get User ("speakers") to work.
Finally, here's the code:
async getAgenda() {
return await Session.aggregate([
{
// this query returns all Sessions...
$match: {
$and: [
{ from: { $gte: new Date("2018-10-22T10:00:00.015Z") } },
{ to: { $lte: new Date("2018-10-22 17:00:00.015Z") } },
]
}
},
{
$lookup: { from: "alldata", localField: "roomId", foreignField: "_id", as: "room" }
},
{
$unwind: { path: '$room' }
},
{
$lookup: { from: "alldata", localField: "speakerIds", foreignField: "_id", as: "spikeri" }
}
])
}
Update: User object (speaker == User):
{
"_id": "5b2ba7784ad0f226d8ae2788",
"group": "USER",
"verified": true,
"eventsRegistered": [
"5b2b77b74ad0f226d8ae2780",
"5b27ca01cc7eff056826acc7"
],
"eventsAttended": [],
"tickets": [
"5b27cc43cc7eff056826accb",
"5b27d284cc7eff056826acd1",
"5b27cceecc7eff056826acd0"
],
"contacts": [],
"meetings": [],
"_type": "User",
"email": "bla#blabla.us",
"password": "...",
"countryCode": "HR",
"firstName": "Blaman",
"lastName": "Bla bla",
"role": "SPEAKER",
"createdAt": "2018-06-21T13:26:16.686Z",
"updatedAt": "2018-07-25T14:21:42.055Z",
"__v": 0
"confirmationCode": "5375159",
"transactionIds": [
"..."
],
"transactionId": "...",
"gender": "",
"jobTitle": "",
"companyAddress": "",
"companyName": "",
"postalCode": "",
"city": "",
"country": "Croatia"
}
Thank you for help in advance!