sqlalchemy 更新column,如果存在则更新，如果不存在，则添加新记录

没有在sqlalchemy 上查询到类似的功能，自己试着这样来实现：

class Friendship(Base):
    __tablename__ = 'friendship'
    
    id = Column(Integer,primary_key=True)
    me = Column(Integer,ForeignKey('users.id'))
    friend = Column(Integer,ForeignKey('users.id'))
    def __init__(self,me,friend):
            self.me = me 
            self.friend = friend
    
    def check_existing(self):
        existing = session.query(Friendship).filter_by(me=self.me,friend=self.friend).first()
        if not existing:
            friendship = Friendship(self.me,self.friend)
        else:
            friendship = existing
        session.close()
        return friendship

使用则如下：
        friendship = Friendship(self.from_user_id,self.friend_id)
        friendship = friendship.check_existing()
        session.add(friendship)
        session.commit()
        session.close()