如何在使用db session时在sql alchemy中使用事务？

Question

我在我的项目中使用sql炼金术,我使用了db session,

engine = create_engine(configuration)
db_session = scoped_session(sessionmaker(autocommit=False,
                                     autoflush=False,
                                     bind=engine))

Base = declarative_base()
Base.query = db_session.query_property()

def init_db():    
    import models
    Base.metadata.create_all(bind=engine)

数据库会话用作:

db_session.merge(order)      #order(model) in object
db_session.commit()

现在我想将数据插入两个表的顺序和订单行项,所以我需要事务,如:1.在第一个插入中我希望插入的顺序的id在第二个插入查询中使用2.如果第二个插入查询失败,那么第一个查询应该回滚

Try:
    #begin transaction/How to begin transaction?
    order=db_session.add(order)      #insert into order
    #is need to commit db_session here as I need inserted orders id
    #here actually db_session.commit() needed to get order's id(auto generated) 
    #if db_session committed here then sql alchemy starts new session       

    order_line_item.id = order.id
    db_session.add(order_line_item)    #insert into order line line item

    db_session.commit()
    #check transaction status if failed then rollback, How to check status?

except:
    db_session.rollback()

如何使用trasaction？

Answer 1

根据Lafada的建议,嵌套交易不适用于这种情况.A flush()会做得很好,例如

db_session.begin()
try:
    db_session.add(order)

    db_session.flush()

    order_line_item.id = order.id
    db_session.add(order_line_item)

    db_session.commit()
except:
    db_session.rollback()

或者更好的是,如果您在订单和订单项之间正确设置了关系,则您甚至不必费心手动分配ID.

Answer 2

您应该使用relationshipSQLAlchemy 的功能,因此您不必使用外键.例如,您的订单商品可能看起来像(我假设您有多方关系):

class OrderLineItem(Base):
    id = Column(Integer, primary_key=True)
    order_id = Column(Integer, ForeignKey('orders.id'))
    order = relationship('Order', backref='order_line_items')

在插入时,您只需分配订单实例:

order_line_item.order = order
session.add(order)  # you even don't have to insert the item!

请参阅教程中的更多详细信息:http://docs.sqlalchemy.org/en/latest/orm/relationships.html