如何在Pyramid中使用SQLAlchemy事件

Question

views.py

def add_post(topic, request):
    post_form = PostForm(request.POST)
    if 'submit' in request.POST and post_form.validate():
        post = Post(body=post_form.body.data)
        post.user = request.user
        post.topic = topic
        DBSession.add(post)
        request.session.flash(_('Post was added'))
        transaction.commit()
        raise HTTPFound(location=request.route_url('topic',id=topic.id))
    return {'post_form':post_form}

models.py

class Topic(Base):
    __tablename__ = 'topics'
    id = Column(Integer, primary_key=True)
    ...
    post_count = Column(Integer, default=0)
    posts = relationship('Post', primaryjoin="Post.topic_id==Topic.id", backref='topic', lazy='dynamic')


class Post(Base):
    __tablename__ = 'posts'
    id = Column(Integer, primary_key=True)
    ...
    topic_id = Column(Integer, ForeignKey('topics.id'))


def post_inserted(mapper, conn, post):
    topic = post.topic
    topic.post_count = topic.posts.count()

event.listen(Post, "after_insert", post_inserted)

我想在我的Pyramid应用中使用SQLAlchemy的“after_insert”事件，来更新Topic模型中属于它的帖子数量。但是我遇到了一个异常：

Traceback (most recent call last):
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/waitress/channel.py", line 329, in service
    task.service()
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/waitress/task.py", line 173, in service
    self.execute()
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/waitress/task.py", line 380, in execute
    app_iter = self.channel.server.application(env, start_response)
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/pyramid/router.py", line 251, in __call__
    response = self.invoke_subrequest(request, use_tweens=True)
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/pyramid/router.py", line 227, in invoke_subrequest
    response = handle_request(request)
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/pyramid_tm/__init__.py", line 107, in tm_tween
    return response
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/transaction/_manager.py", line 116, in __exit__
    self.commit()
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/transaction/_manager.py", line 107, in commit
    return self.get().commit()
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/transaction/_transaction.py", line 354, in commit
    reraise(t, v, tb)
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/transaction/_transaction.py", line 345, in commit
    self._commitResources()
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/transaction/_transaction.py", line 493, in _commitResources
    reraise(t, v, tb)
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/transaction/_transaction.py", line 465, in _commitResources
    rm.tpc_begin(self)
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/zope/sqlalchemy/datamanager.py", line 86, in tpc_begin
    self.session.flush()
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/sqlalchemy/orm/session.py", line 1583, in flush
    self._flush(objects)
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/sqlalchemy/orm/session.py", line 1654, in _flush
    flush_context.execute()
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/sqlalchemy/orm/unitofwork.py", line 331, in execute
    rec.execute(self)
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/sqlalchemy/orm/unitofwork.py", line 475, in execute
    uow
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/sqlalchemy/orm/persistence.py", line 67, in save_obj
    states_to_insert, states_to_update)
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/sqlalchemy/orm/persistence.py", line 702, in _finalize_insert_update_commands
    mapper.dispatch.after_insert(mapper, connection, state)
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/sqlalchemy/event.py", line 291, in __call__
    fn(*args, **kw)
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/sqlalchemy/orm/events.py", line 360, in wrap
    wrapped_fn(*arg, **kw)
  File "/home/user/workspace/myforum/cube_forum/models.py", line 165, in post_saved
    topic.post_count = topic.posts.count()
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/sqlalchemy/orm/dynamic.py", line 249, in count
    sess = self.__session()
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/sqlalchemy/orm/dynamic.py", line 219, in __session
    sess.flush()
  File "/home/user/workspace/myforum/env/lib/python2.6/site-packages/sqlalchemy/orm/session.py", line 1577, in flush
    raise sa_exc.InvalidRequestError("Session is already flushing")
InvalidRequestError: Session is already flushing

在Pyramid/SQLAlchemy中怎么才能正确做到这一点呢？

补充说明：其实我的问题是如何在Pyramid中使用SQLAlchemy的事件。

Answer 1

我发现数据库层面有个问题——我们不能保证得到的帖子数量是有效的。想象一下，如果有两个或更多的帖子同时被添加到一个话题中，而这些操作是分开进行的，那么我们就无法准确知道这个话题下有多少个有效的帖子，除非你在每次添加新帖子时都锁定话题表或者记录。

可能的解决办法：

1) 在话题类中添加一个方法。这个方法总是能返回有效的帖子数量，但每次调用时都会向数据库查询一次。

class Topic(Base):
    ...
    def posts_count(self):
        return self.posts.count()

2) 在话题中添加帖子，并在你的代码中更新帖子数量，而不是使用插入后事件。每次你更改帖子的话题时，都需要更新话题的帖子数量。不过，这个方法也无法解决我之前提到的，当多个帖子同时添加到话题时的问题。

def add_post(topic, request):
    post_form = PostForm(request.POST)
    if 'submit' in request.POST and post_form.validate():
        post = Post(body=post_form.body.data)
        post.user = request.user

        topic.posts.append(post)
        topic.post_count = topic.posts.count()

        DBSession.add(post)
        DBSession.add(topic)

        request.session.flash(_('Post was added'))
        transaction.commit()
        raise HTTPFound(location=request.route_url('topic',id=topic.id))
    return {'post_form':post_form}

Answer 2

SQLAlchemy 的 "after_insert" 事件不适合这个任务。

解决办法是使用 Pyramid 的自定义事件，具体可以参考这里 http://dannynavarro.net/2011/06/12/using-custom-events-in-pyramid/