Get only the length of mongoose virtual populate - database

Question

I would like to know if there is a way in Mongoose of getting the number of matching localField/foreignField as virtual field, without retrieving all the documents through virtual population.
Example:
const ForumThreadSchema = new Schema({
code: { type: Number, required: true, unique: true },
title: { type: String, min: [10, 'Too short title'], max: [200, 'Too long title'], required: true },
description: { type: String, min: [10, 'Too short description'], max: [2000, 'Too long description'], required: true },
creation_date: { type: Date, default: Date.now, required: true },
updated_date: { type: Date, default: Date.now, required: true },
_authorId: { type: Schema.ObjectId, ref: 'User', required: true },
_forumId: { type: Schema.ObjectId, ref: 'Forum', required: true }
}, {
collection: 'ForumThreads',
toObject: { virtuals: true },
toJSON: { virtuals: true }
});
const ForumMessageSchema = new Schema({
code: { type: Number, required: true, unique: true },
content: { type: String, min: [10, 'Too short message content'], max: [2000, 'Too long message content'], required: true },
creation_date: { type: Date, default: Date.now, required: true },
update_date: { type: Date, default: Date.now, required: true },
_authorId: { type: Schema.ObjectId, ref: 'User', required: true },
_forumThreadId: { type: Schema.ObjectId, ref: 'ForumThread', required: true },
_parentMessageId: { type: Schema.ObjectId, ref: 'ForumMessage' }
}, {
collection: 'ForumMessages'
});
The virtual populate on the forum thread schema retrieves me all the message documents. I need a virtual field with only their number if possible.
ForumThreadSchema
.virtual('messages', {
ref: 'ForumMessage',
localField: '_id',
foreignField: '_forumThreadId'
});
ForumThreadSchema
.virtual('messages_count')
.get(function() {
return this.messages.length;
});
The second virtual only works if the population of the first one is done.
I also want to mantain the virtual populate but I would like to find a way of getting the number of matching documents without use it (in server APIs that not need all message documents but only their size).
Is it possible?

Answer 1

You just need to add the count option:
ForumThreadSchema
.virtual('messages_count', {
ref: 'ForumMessage',
localField: '_id',
foreignField: '_forumThreadId',
count: true
});
And then when you can populate just the count:
ForumThread.find({}).populate('messages_count')
Doc: https://mongoosejs.com/docs/populate.html#count