当 expire_on_commit=True 时，commit 之后所有实例都会过期，之后再访问这些过期实例的属性时，SQLAlchemy 会重新去数据库加载实例对应的数据记录。
# SQLAlchemy 源码，非关键内容省略
class Session(_SessionClassMethods):
"""
Manages persistence operations for ORM-mapped objects.
The Session's usage paradigm is described at :doc:`/orm/session`.
"""
...
def __init__(
self,
bind=None,
autoflush=True,
expire_on_commit=True,
...
):
"""
:param expire_on_commit: Defaults to ``True``. When ``True``, all
instances will be fully expired after each :meth:`~.commit`,
so that all attribute/object access subsequent to a completed
transaction will load from the most recent database state.
"""