Postgresql sqlalchemy default time now()一次又一次地提供相同的时间

from sqlalchemy import create_engine, Column, DateTime
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.sql import func

# 创建数据库连接
engine = create_engine('postgresql://username:password@localhost/dbname')

# 创建基类
Base = declarative_base()

# 定义表模型
class MyTable(Base):
    __tablename__ = 'my_table'
    id = Column(Integer, primary_key=True)
    created_at = Column(DateTime, default=func.now())

# 创建表
Base.metadata.create_all(engine)