Question

我在使用 SQLAlchemy / Tastypie REST 界面插入数据库项时出现问题，但随后获取项列表时缺少该项。只有在我获得第二次的项目列表后才会显示。

我正在使用SQLAlchemy和Tastypie / Django通过mod_wsgi在Apache上运行。我使用单独的数据库管理器类来保存我的引擎和declarative_base，并使用Tastypie，一个单独的类来获取会话，并确保如果提交有问题我回滚。如下面的更新，插入后我不关闭我的会话时会出现问题。 为什么这有必要？

我的原始代码是这样的：

Session = scoped_session(sessionmaker(autoflush=True))

# Singleton Database Manager class for managing session
class DatabaseManager():
    engine = None
    base = None

    def ready(self):
       host='mysql+mysqldb://etc...'
       if self.engine and self.base:
            return True
        else:
            try:
                self.engine = create_engine(host, pool_recycle=3600)
                self.base = declarative_base(bind=self.engine)
                return True
            except:
                return False

    def getSession(self):
        if self.ready():
            session = Session()
            session.configure(bind=self.engine)
            return session
        else:
            return None

DM = DatabaseManager()

# A session class I use with Tastypie to ensure the session is destroyed at the
# end of the transaction, because Tastypie creates singleton Resources used for
# all threads
class MySession:
    def __init__(self):
        self.s = DM.getSession()

    def safeCommit(self):
        try:
            self.s.commit()
        except:
            self.s.rollback()
            raise

    def __del__(self):
        try:
            self.s.commit()
        except:
            self.s.rollback()
            raise


# ... Then ... when I get requests through Apache/mod_wsgi/Django/Tastypie
# First Request
obj_create():
    db = MySession()
    print db.s.query(DBClass).count() # returns 4
    newItem = DBClass()
    db.s.add(newItem)
    db.s.safeCommit()
    print db.s.query(DBClass).count() # returns 5


# Second Request after First Request returns
obj_get_list():
    db = MySession()
    print db.s.query(DBClass).count() # returns 4 ... should be 5


# Third Request is okay
obj_get_list():
    db = MySession()
    print db.s.query(DBClass).count() # returns 5

更新

进一步挖掘后，问题似乎是我的会话需要在创建后关闭。也许是因为Tastypie的object_create（）将SQLAlchemy对象添加到它的bundle中，我不知道它离开函数范围后会发生什么：

obj_create():
    db = MySession()
    newItem = DBClass()
    db.s.add(newItem)
    db.s.safeCommit()
    copiedObj = copyObj(newItem) # copy SQLAlchemy record into non-sa object (see below)
    db.s.close()
    return copiedObj

如果有人在回答中解释这一点，我可以提出问题。另外，对于那些好奇的人，我将我的对象从SQLAlchemy中复制出来：

class Struct:
    def __init__(self, **entries):
        self.__dict__.update(entries)

class MyTastypieResource(Resource):
    ...
    def copyObject(self, object):
        base = {}
        # self._meta is part of my tastypie resource
        for p in class_mapper(self._meta.object_class).iterate_properties:
            if p.key not in base and p.key not in self._meta.excludes:
                base[p.key] = getattr(object,p.key)
        return Struct(**base)

Answer 1

关闭我的会话解决了问题。答案中的更新并没有完全解决问题 - 我最终添加了一个中间件类来在事务结束时关闭会话。这确保了所有内容都写入数据库。中间件看起来有点像这样：

class SQLAlchemySessionMiddleWare(object):
    def process_response(self, request, response):
        try:
            session = MyDatabaseManger.getSession()
            session.commit()
            session.close()
        except Exception, err:
            pass
        return response

    def process_exception(self, request, exception):
        try:
            session = MyDatabaseManger.getSession()
            session.rollback()
            session.close()
        except Exception, err:
            pass

SQLAlchemy查询不在备用会话中检索已提交的数据

1 个答案: