Django ORM: Get first instance for each foreignkey - sql

Question

I have the following models:
class Author(models.Model):
name = models.CharField(max_length=100)
class Book(models.Model):
title = models.CharField(max_length=100)
author = models.ForeignKey(Author, on_delete=models.CASCADE)
number = models.IntegerField()
Some Authors might have no Book. Is there a way to get a list or set containing exactly one Book per Author who wrote at least on Book? I'm looking for a solution in one single SQL transaction.
For example, if I have the following entries:
Authors:
Albert Camus
Friedrich Nietzsche
Sigmund Freud
Books:
Thus Spoke Zarathustra by Nietzsche
The Myth of Sisyphus by Camus
The Rebel by Camus
I want a query which returns [Thus Spoke Zarathustra, The Myth of Sisyphus], or [Thus Spoke Zarathustra, The Rebel].
Bonus points of the query returns the books with the lowest number.

Answer 1

You should be able to achieve this using a Subquery - for example
from django.db.models import OuterRef, Subquery
books = Book.objects.filter(author_id=OuterRef('pk')).order_by('number')
authors = Author.objects.annotate(book_title=Subquery(books.values('title')))
for author in authors:
print(author.name, author.book_title)

Answer 2

You'll have to use raw queries. Something like this:
query='''
SELECT b1.id, b1.title, b1.author_id, b1.number from Books_book b1, (
SELECT author_id, min(number) as min_number
from Books_book
GROUP BY author_id
) as b2
WHERE b1.author_id=b2.author_id AND b1.number = b2.min_number
'''
books_list = Book.objects.raw(query)[:]
Now books_list contains one book for each author(with lowest number), as required