SqlAlchemy + Mysql:由于IntegrityError而无法删除表

Question

当我尝试重新初始化我的数据库时,我不断收到IntegrityError:

sqlalchemy.exc.IntegrityError: (IntegrityError) (1217, 'Cannot delete or update a parent row: a foreign key constraint fails') '\nDROP TABLE users' ()

在我的init_db脚本中,我有:

def main(argv=sys.argv):
    if len(argv) != 2:
        usage(argv)
    config_uri = argv[1]
    setup_logging(config_uri)
    settings = get_appsettings(config_uri)
    engine = create_engine('...')
    Session.configure(bind=engine)
    session = Session()
    Base.metadata.bind=engine
    Base.metadata.drop_all(engine)
    Base.metadata.create_all(engine)

它会让我删除Users表,这是:

class User(Base):
    __tablename__='users'
    __table_args__={
        'mysql_engine':'InnoDB',
        'mysql_charset':'utf8',
    }

    user_id = Column(Integer, primary_key=True, unique=True)
    username = Column(VARCHAR(16), primary_key=True, unique=True)
    name = Column(VARCHAR(16))
    password = Column(VARCHAR(20))
    email = Column(VARCHAR(30))
    creation = Column(DateTime, default = datetime.datetime.now)    

    def __init__(self, un, name, pw, email):
        self.username = un
        self.name = name
        self.email = email
        self.password = pw

Answer 1

我看到两个可能的原因:

1. 未映射表:你必须具有数据库的另一个表ForeignKey的users表,但不映射到任何sqlalchemy模型,因此在不掉线drop_all操作.
2. 缺少ForeignKey映射:sqlalchemy模型中缺少某些关系.例如,addressesDB上的表有一个ForeignKey指向users表的表,但是你的模型Address(或正确地说,它的映射器Address)不配置这个ForeignKey,因此在drop_all sqlalchemy没有办法知道这个表应该是在引用的one(users)之前删除.