SQLAlchemy+Tornado: How to create a scopefunc for

问题:

Using tornado, I want to create a bit of middleware magic that ensures that my SQLAlchemy sessions get properly closed/cleaned up so that objects aren't shared from one request to the next. The trick is that, since some of my tornado handlers are asynchronous, I can't just share one session for each request.

So I am left trying to create a ScopedSession that knows how to create a new session for each request. All I need to do is define a scopefunc for my code that can turn the currently executing request into a unique key of some sort, however I can't seem to figure out how to get the current request at any one point in time (outside of the scope of the current RequestHandler, which my function doesn't have access to either).

Is there something I can do to make this work?

回答1:

You might want to associate the Session with the request itself (i.e. don't use scopedsession if it's not convenient). Then you can just say, request.session. Still needs to have hooks at the start/end for setup/teardown.

edit: custom scoping function

def get_current_tornado_request():
   # TODO: ask on the Tornado mailing list how
   # to acquire the request currently being invoked

Session = scoped_session(sessionmaker(), scopefunc=get_current_tornado_request)

回答2:

(This is a 2017 answer to a 2011 question) As @Stefano Borini pointed out, easiest way in Tornado 4 is to just let the RequestHandler implicitly pass the session around. Tornado will track the handler instance state when using coroutine decorator patterns:

import logging

_logger = logging.getLogger(__name__)

from sqlalchemy import create_engine, exc as sqla_exc
from sqlalchemy.orm import sessionmaker, exc as orm_exc

from tornado import gen
from tornado.web import RequestHandler

from my_models import SQLA_Class

Session = sessionmaker(bind=create_engine(...))

class BaseHandler(RequestHandler):

    @gen.coroutine
    def prepare():
        self.db_session = Session()

    def on_finish():
        self.db_session.close()

class MyHander(BaseHandler):

    @gen.coroutine
    def post():
        SQLA_Object = self.db_session.query(SQLA_Class)...
        SQLA_Object.attribute = ...

        try:
            db_session.commit()
        except sqla_exc.SQLAlchemyError:
            _logger.exception("Couldn't commit")
            db_session.rollback()

If you really really need to asynchronously reference a SQL Alchemy session inside a declarative_base (which I would consider an anti-pattern since it over-couples the model to the application), Amit Matani has a non-working example here.