I'm using Mongoose and a MongoDB. I have three schemas user and genres and songs.

const userSchema = new Schema({
  name: String,
  age: String,
  favorite_genres: {
    type: Schema.Types.ObjectId,
    ref: 'Genre'
  }
});

const genreSchema = new Schema({
  name: String,
  songs: {
    type: Schema.Types.ObjectId,
    ref: 'Song'
  }
});

const songSchema = new Schema({
  title: String,
  artist: String,
  length: String,
  year: String
});

those are simplifid schemas, but they will do for this puprose. So when I query a user I want to populate the doc with the favorite genres. But there (due to the potential amount of data) not with the ids of all songs, but only the number of songs. How can I achieve that?

I tried virtuals but they don't work the way I wanted it. What I'm looking for is sth like the following:

const user = await User.findById(userID).populate({path: 'favorite_genres', select: 'name', sizeof: 'songs'}).lean()

// or

const user = await User.findById(userID).populate({path: 'favorite_genres', select: ['name', 'songs.length']}).lean()

Does anyone have an idea?