在SQLAlchemy中定义表时，如何将函数(表达式依赖于其他列)定义为列的默认值？

from sqlalchemy import create_engine, Column, Integer, String, func
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()

class MyTable(Base):
    __tablename__ = 'my_table'
    id = Column(Integer, primary_key=True)
    name = Column(String(50))
    created_at = Column(Integer, default=func.now())

engine = create_engine('sqlite:///mydatabase.db')
Base.metadata.create_all(engine)