如何将一个ponyorm实体属性与其他几个实体关联起来？

from pony.orm import *

db = Database()

class Category(db.Entity):
    name = Required(str)
    products = Set('Product')

class Product(db.Entity):
    name = Required(str)
    category = Required(Category)

db.bind(provider='sqlite', filename=':memory:')
db.generate_mapping(create_tables=True)

with db_session:
    category = Category(name='Electronics')
    product1 = Product(name='Laptop', category=category)
    product2 = Product(name='Smartphone', category=category)

    commit()

# 查询某个分类下的所有产品
with db_session:
    category = Category.get(name='Electronics')
    products = category.products
    for product in products:
        print(product.name)