Django queryset to return first of each item in foreign key based on date

need to get a queryset with the first book (by a date field) for each author (related to by foreign key) ...is there a Django ORM way to do this (without custom SQL preferred but acceptable)

*Edit: Please note that an answer that works using only a modern open source backend like Postgresql is acceptable ..still ORM based solution preferred over pure custom sql query)

Models
class Book(Model):
  date = Datefield()
  author = ForeignKey(Author)

class Author(Model):
  name = CharField()


Book.objects.filter(??)