Question

我的模型（之前）包含客户端默认值：

created_ts = db.Column(db.DateTime(timezone=True), default=dt.datetime.now)

我的模型（之后）现在包含服务器端默认值：

 created_ts = db.Column(db.DateTime(timezone=True), server_default=text('NOW()'))

但是，我现在开始看到错误：

InvalidRequestError: This session is in 'committed' state; no further SQL can be emitted within this transaction.

在我的models_committed挂钩中：

@models_committed.connect_via(app)
def handle(sender, changes):
    for model, operation in changes:
        model.to_dict() # error here

我从flask_sandboy偷走了to_dict：

def to_dict(self):
    """Return dict representation of class by iterating over database
    columns."""
    value = {}
    for column in self.__table__.columns:
        attribute = getattr(self, column.name) # error here
        if isinstance(attribute, datetime.datetime):
            attribute = str(attribute)
        value[column.name] = attribute
    return value

所以，getattr(self, column.name)似乎以某种方式触发服务器端默认值（可能是因为这是我引入的更改）。

在我自己的代码中的这一行，我提供了堆栈跟踪的其余部分：

  File "/code/models/session.py", line 20, in to_dict
    attribute = getattr(self, column.name)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/attributes.py", line 239, in __get__
    return self.impl.get(instance_state(instance), dict_)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/attributes.py", line 589, in get 
    value = callable_(state, passive)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/state.py", line 433, in __call__
    self.manager.deferred_scalar_loader(self, toload)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/loading.py", line 613, in load_scalar_attributes
    only_load_props=attribute_names)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/loading.py", line 235, in load_on_ident
    return q.one()
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2398, in one 
    ret = list(self)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2441, in __iter__
    return self._execute_and_instances(context)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2454, in _execute_and_instances
    close_with_result=True)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/query.py", line 2445, in _connection_from_session
    **kw)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 880, in connection
    execution_options=execution_options)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 885, in _connection_for_bind
    engine, execution_options)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 305, in _connection_for_bind
    self._assert_active()
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 196, in _assert_active
    "This session is in 'committed' state; no further "
InvalidRequestError: This session is in 'committed' state; no further SQL can be emitted within this transaction.

如何解决此问题？

pip freeze：

aniso8601==0.92
blinker==1.3
boto==2.36.0
Flask==0.10.1
Flask-Cors==1.10.3
Flask-HTTPAuth==2.4.0
Flask-SQLAlchemy==2.0
gunicorn==19.3.0
itsdangerous==0.24
psycopg2==2.6
pytz==2014.10
six==1.9.0
SQLAlchemy==0.9.9
Werkzeug==0.10.1

Answer 1

TL; DR解决方案＃4正常工作

created_ts字段在该请求线程中以及后续信号处理程序中是未知的，因为该值是由PostgresQL在服务器端选择的。

getattr(self, column.name)然后尝试 ~~hydrate~~ 通过返回数据库服务器来填充该值，但遗憾的是现在不在事务中。

这里有几个选项：

在信号处理程序中启动一个新会话，以便从数据库中检索值（这是一个未经测试的建议）
使用内置的_dict获取模型的状态，但它不会拥有created_ts字段，因为此时尚不知道：

代码

modeldict = dict(model.__dict__)
modeldict.pop('_sa_instance_state', None)

坚持使用default=dt.datetime.now，但写入数据库的非Python应用需要提供created_ts。

因此，权衡是模型的消费者（与pika评论一起提到）是否需要created_ts。

（经过测试开始工作）感谢#sqlalchemy IRC会议室的 agronholm 和this ticket kbussel 的建议我试图在会话仍处于打开状态时序列化数据，当我确定数据已提交时，我重新使用序列化数据并绕过返回数据库：

from sqlalchemy.event import listens_for
from flask.ext.sqlalchemy import SignallingSession

@listens_for(SignallingSession, 'after_flush')
def after_flush_handler(session, tx):
        try:
            d = session._model_changes
        except AttributeError:
            return

        if d:
            changes = []
            for model, operation in list(d.values()):
                changes.append((model.to_dict(), operation))
            session.info['my_changes'] = changes
            d.clear()


@listens_for(SignallingSession, 'after_commit')
def after_commit_handler(session):
        if 'my_changes' in session.info:
            changes = session.info['my_changes']
            for model, operation in changes:
                # use model here, with all data populated

在models_committed信号处理程序中的模型中使用server_default时，Flask SQLAlchemy Transaction错误

1 个答案: