Mongodb select 'field' with condition in Query Population - mongodb

Question

Let's say that I have three entities Track, Chapter and Episode, a track can contain one or more chapters, a chapter is associated with one track and can contain zero or more episodes.
Track.ts :
import { Prop, Schema, SchemaFactory } from '#nestjs/mongoose';
import { Document } from 'mongoose';
import * as mongoose from 'mongoose';
import { Chapter } from './chapter.entity';
#Schema({
timestamps: true,
})
export class Track extends Document {
...
#Prop({
type: [mongoose.Schema.Types.ObjectId],
ref: 'Chapter',
})
chapters: [Chapter];
}
export const TrackSchema = SchemaFactory.createForClass(Track);
Chapter.ts :
import { Prop, Schema, SchemaFactory } from '#nestjs/mongoose';
import { Document } from 'mongoose';
import * as mongoose from 'mongoose';
import { Track } from './track.entity';
import { Episode } from './episode.entity';
#Schema({
timestamps: true,
})
export class Chapter extends Document {
...
#Prop({
type: mongoose.Schema.Types.ObjectId,
ref: 'Track',
required: true,
})
track: Track;
#Prop({
type: [mongoose.Schema.Types.ObjectId],
ref: 'Episode',
})
episodes: [Episode];
}
export const ChapterSchema = SchemaFactory.createForClass(Chapter);
What I want is to find a "track" and populate its "chapters" but without selecting the field "episodes" if it is an empty array. I was looking in the docs I found out how to populate documents with condition but nothing about fields, I found out also how to select some fiels but not with condition.
var track = await this.trackModel
.findById(id)
.populate({
path: 'chapters',
select: '-episodes', // this excludes the episodes field, how can I do it with condition
})
.lean();
Thank you for your attention!