MongoDB + express.js + Mongoose - Aggregate pipeline instead of populate

book.js

const Author = require('../models/author')
const Book = require('../models/book')
//...

const getAll = async ({ author, genre }) => {
  let bookAggregate = Book.aggregate()

  if (genre) {
    bookAggregate = bookAggregate.match({ genres: genre })
  }

  bookAggregate = bookAggregate
    .lookup({
      from: Author.collection.name,
      as: 'author',
      localField: 'author',
      foreignField: '_id'
    })
    .unwind('author')

  if (author) {
    bookAggregate = bookAggregate.match({ 'author.name': author })
  }

  return await bookAggregate.exec()
}

// ...

module.exports = {
  getAll,
  // ...
}