flush가 필요한 이유: https://stackoverflow.com/questions/4201455/sqlalchemy-whats-the-difference-between-flush-and-commit

https://dev.to/uponthesky/python-post-reviewhow-to-implement-a-transactional-decorator-in-fastapi-sqlalchemy-ein

https://medium.com/@ramanbazhanau/mastering-sqlalchemy-a-comprehensive-guide-for-python-developers-ddb3d9f2e829