How to use SQLAlchemy's events in Pyramid

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)

I want in my Pyramid app to use SQLAchemy event 'after_insert', to update Topic model with number of posts belong to it. But I get exeption:

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

How to do it right in Pyramid/SQLalchemy?

EDIT: Question is actually how to use SQLAlchemy's events in Pyramid.

Answer 1

SQLAlchemy event "after_insert" not suitable for this task.

The answer is using Pyramid's custom events as described here http://dannynavarro.net/2011/06/12/using-custom-events-in-pyramid/

Answer 2

I see that there is a problem on the database level - there no guarantee to get valid posts count value. Imagine if two or more posts were added to topic at the same time within multiple transactions - there is no way to tell for sure valid count of associated posts unless you will lock topic table or record every time you are adding new post.

Possible solutions:

1) Add method to Topic class. Always valid number, but it will execute query into db every time.

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

2) Add post to topic and update post_count in your code without using after_insert event. You'll need to update topic.post_count every time when you change topic for post. Also this method won't resolve issue I described when several posts were added to topic at the same time.

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}