MongoDB Query solution - express

Question

What is the optimize Query for this situation
So the Situation is a User is following many XY user and these XY have got events, So what will the best and optimize query to get all the events from his followers XY in sorted form (sort by Date). I have got create Date in my schema
This is my User Schema
var userSchema = new Schema({
followers:[{
type: Schema.Types.ObjectId,
ref: 'XY'
}]
});
var User = mongoose.model('User',userSchema);
module.exports = User;
Here is My XY schema
var XY= new Schema({
events:[{
type: Schema.Types.ObjectId,
ref: 'Event'
}],
});
var XY= mongoose.model('XY',XY);
module.exports = XY;

Answer 1

Try this.
User.aggregate([
{ $match: { _id: mongoose.Types.ObjectId(<userId here>) } },
{ $unwind: { path: "$followers" } },
{ $lookup: { from: 'XY', localField: 'followers', foreignField: '_id', as: 'followers' } },
{ $unwind: { path: "$followers" } },
{ $unwind: { path: "$followers.events" } },
{ $lookup: { from: 'Event', localField: 'followers.events', foreignField: '_id', as: 'followers.events' } },
{ $unwind: { path: "$followers.events" } },
{ $sort: { "followers.events.createdDate": **-1** } }, // -1 -> desc, 1 -> asc
{
"$project":
{
"_id": "$followers.events._id",
"createdDate": "$followers.events.createdDate",
// populate other event details here accordingly
}
}
], function (err, events, next) {
});
$lookup lets you populate a sub-document from a different schema.
After populating, the resultant documents will be an array, so $unwind is used before working on them.
note that $unwind is also used before doing a $lookup here as the field we are trying to populate is itself an array.