SQLAlchemy+Tornado: How to create a scopefunc for SQLAlchemy's ScopedSession?

Question

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 req

Answer 1

(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.