class Book(Model):
    released_at=DateTimeField()

class BookPrice(Model):
    price = DecimalField()
    created_at = DateTimeField()

books = Book.objects.annotate(
            old_price=Subquery(BookPrice.objects.filter(
                          created_at__lt=OuterRef('released_at')
                          )
                          .order_by('created_at')
                          .last()
                          .price
                      )
           )