在PonyORM中找不到反向属性

from pony.orm import *

db = Database()

class Author(db.Entity):
    name = Required(str)
    books = Set("Book")

class Book(db.Entity):
    title = Required(str)
    author = Required(Author)

db.bind("sqlite", ":memory:")
db.generate_mapping(create_tables=True)

with db_session:
    author = Author(name="John Smith")
    book1 = Book(title="Book 1", author=author)
    book2 = Book(title="Book 2", author=author)

# 查询特定作者的所有书籍
with db_session:
    author = Author.get(name="John Smith")
    books = select(book for book in Book if book.author == author)[:]
    print(books)