Do an aggregate with a populate - mongodb

Question

I'm having troubles with the following. I wonder if it's possible to do it with a single query.
So I have the following model :
const Analytics = new Schema({
createdAt: {
type: Date,
default: Moment(new Date()).format('YYYY-MM-DD')
},
loginTrack: [
{
user_id: {
type: mongoose.Schema.Types.ObjectId,
ref: 'Users',
}
}
]
}, { collection: 'analytics' });
And the user model :
const UserSchema = new mongoose.Schema(
{
nickname: {
type: String,
required: true,
unique: true
},
instance: {
type: mongoose.Schema.Types.ObjectId,
ref: 'Instances',
default: null
}}, {collection: 'users'});
I want to get the connected users for a specific instance at a specific date.
AnalyticsModel.aggregate([
{
$match: {
createdAt: { "$gte": moment(args.startDate).format('YYYY-MM-DD'), "$lt": moment(args.endDate).format('YYYY-MM-DD')}
}
},
{
"$project": {
users: { $size: "$loginTrack" },
"createdAt": 1,
"_id": 0
}
}, {
"$group": {
"_id": "$createdAt",
"count": { "$sum": "$users" }
}
}
This gets me
[ { _id: '2019-02-11', count: 3 },
{ _id: '2019-02-08', count: 6 },
{ _id: '2019-02-07', count: 19 },
{ _id: '2019-02-06', count: 16 } ]
The results expected will be the same but I want to filter on users that belongs to a specific instance
Is it possible to do it with a single query or I need to do a populate first before the aggregation ?
UPDATE
I did some progress on it, I needed to add a lookup and I think it's ok :
AnalyticsModel.aggregate([
{"$unwind": "$loginTrack"},
{
$lookup:
{
from: 'users',
localField:'loginTrack.user_id',
foreignField: '_id',
as: '_users'
}
},
{
$match: {
createdAt: { "$gte": new Date(args.startDate), "$lt": new Date(args.endDate)}
}
},
{
$project: {
_users: {
$filter: {
input: '$_users',
as: 'item',
cond: {
$and: [
{ $eq: ["$$item.instance", new ObjectId(args.instance_id)] }
]
}
}
},
"createdAt": 1,
"_id": 0
}
},
{
"$group": {
"_id": "$createdAt",
"count": { "$sum": { "$size": "$_users" } }
}
}
Also the dates were in string in the model.
The output is now :
[ { _id: 2019-02-11T00:00:00.000Z, count: 2 } ]