How to select rows in Mongodb based on row values ​​of a related collection?

Question

MongoDB

MongoDB: #Collection
Authors
db.authors.insert({name: 'Chehov', birthday: '1860', country: 'ru'})
db.authors.insert({name: 'Sartr', birthday: '1905', country: 'fr'})
db.authors.insert({name: 'Braun', birthday: '1913', country: 'us'}
... #Book
Collection
db.books.insert({title: 'book1' , author: 'Chehov', year :''})
db.books.insert({title: 'book2', author: 'Sartr', year :''})
db.books.insert({title: 'book3' , author: 'Frauthor', year :''})
...
Let's assume that the Authors collection will be expanded with fields, that is, new fields will be added - some tags, for example.that the option to update the structure of the Books collection is not suitable.
What is the correct (productive) way to select all books that have country == 'fr' field, for example?
The question is about MongoDB, not about which database is better to use.

Answer 1

Here, probably, only "on the forehead", the first request is to get all the necessary authors, the second - to get books by these authors. It is also necessary that there were necessary indexes.
If the operation is frequent, then it is better to change the structure / make caching (for example, make books.author_country)