Getting related data with NestJS and Mongo - mongodb

Question

I'm new to NestJS and Mongo. I'm defining the schemes and I'm trying to get the data of related collections. I followed the examples from here but I don't get it why is not working in my code. I saw another example in StackOverflow with no success.
What I get:
[
{
"songs": [],
"_id": "6066d3783b1f96201a62372a",
"name": "The best album",
"year": 2021,
"image": "cover"
}
]
What I'm trying to get:
[
{
"songs": [
{
"_id": "6066d3e13b1f96201a62372b",
"name": "Song1"
},
{
"_id": "6066d46a3b1f96201a62372c",
"name": "Song2"
},
],
"_id": "6066d3783b1f96201a62372a",
"name": "The best album",
"year": 2021,
"image": "cover"
}
]
Album Document
{
"_id": {
"$oid": "6066d3783b1f96201a62372a"
},
"name": "The best album",
"year": {
"$numberInt": "2021"
},
"image": "cover"
}
Song Document
{
"_id": {
"$oid": "6066d3e13b1f96201a62372b"
},
"name": "Song1",
"album": {
"$oid": "6066d3783b1f96201a62372a"
}
},
{
"_id": {
"$oid": "6066d46a3b1f96201a62372c"
},
"name": "Song2",
"album": {
"$oid": "6066d3783b1f96201a62372a"
}
}
album.module.ts
import { Module } from '#nestjs/common';
import { AlbumsController } from './albums.controller';
import { AlbumsService } from './albums.service';
import { MongooseModule } from '#nestjs/mongoose';
import { Album, AlbumSchema } from './schemas/album.schema';
#Module({
imports: [MongooseModule.forFeature([{ name: Album.name, schema: AlbumSchema }])],
controllers: [AlbumsController],
providers: [AlbumsService],
})
export class AlbumModule {}
albums.controller.ts
import { Controller, Get, HttpStatus, Res } from '#nestjs/common';
import { AlbumsService } from './albums.service';
#Controller('albums')
export class AlbumsController {
constructor(private readonly albumsService: AlbumsService) {}
#Get()
async getAlbums(#Res() res) {
const albums = await this.albumsService.getAlbums();
return res.status(HttpStatus.OK).json(albums);
}
}
albums.service.ts
import { Model } from 'mongoose';
import { Injectable } from '#nestjs/common';
import { InjectModel } from '#nestjs/mongoose';
import { Album, AlbumDocument } from './schemas/album.schema';
#Injectable()
export class AlbumsService {
constructor(#InjectModel(Album.name) private albumModel: Model<AlbumDocument>) {}
async getAlbums(): Promise<Album[]> {
return this.albumModel.find().populate('songs').exec();
}
}
album.schema.ts
import { Prop, Schema, SchemaFactory } from '#nestjs/mongoose';
import { Song } from './song.schema';
import { Document, Types } from 'mongoose';
import * as mongoose from 'mongoose';
export type AlbumDocument = Album & Document;
#Schema()
export class Album {
#Prop()
name: string;
#Prop()
year: number;
#Prop()
image: string;
#Prop([{ type: mongoose.Schema.Types.ObjectId, ref: 'Song' }])
songs: Song[];
}
export const AlbumSchema = SchemaFactory.createForClass(Album);
song.schema.ts
import { Prop, Schema, SchemaFactory } from '#nestjs/mongoose';
import { Album } from './album.schema';
import { Document } from 'mongoose';
import * as mongoose from 'mongoose';
export type SongDocument = Song & Document;
#Schema()
export class Song {
#Prop()
name: string;
#Prop({ type: mongoose.Schema.Types.ObjectId, ref: 'Album' })
album: Album;
}
export const SongSchema = SchemaFactory.createForClass(Song);
Thank you in advance.