SQLAlchemy and Falcon - session initialization

Question

I'm wondering where the best place would be to create a scoped session for use in falcon.

From reading the flask-sqlalchemy code, it, in a round about way, does something like this:

from sqlalchemy import create_engine
from sqlalchemy.orm import scoped_session, sessionmaker

try:
    from greenlet import get_current as get_ident
except ImportError:
    try:
        from thread import get_ident
    except ImportError:
        from _thread import get_ident

connection_uri = 'postgresql://postgres:@localhost:5432/db'
engine = create_engine(connection_uri)
session_factory = sessionmaker(bind=engine)
session_cls = scoped_session(session_factory, scopefunc=get_ident)
session = session_cls()

Would this work for falcon? Will the get_ident func "do the right thing" when using gunicorn?

Waw

Answer 1

You can use middleware

Example.

1. Create engine, session_factory and scoped_session object.

   from sqlalchemy import create_engine
   from sqlalchemy.orm import scoped_session
   from sqlalchemy.orm import sessionmaker
   
   import settings
   
   
   engine = create_engine(
       '{engine}://{username}:{password}@{host}:{port}/{db_name}'.format(
       **settings.POSTGRESQL
       )
   )
   
   session_factory = sessionmaker(bind=engine)
   Session = scoped_session(session_factory)
   

2. Create middleware.

   class SQLAlchemySessionManager:
       """
       Create a scoped session for every request and close it when the request
       ends.
       """
   
       def __init__(self, Session):
           self.Session = Session
   
       def process_resource(self, req, resp, resource, params):
           resource.session = self.Session()
   
       def process_response(self, req, resp, resource, req_succeeded):
           if hasattr(resource, 'session'):
               Session.remove()
   

3. Register middleware.

   import falcon
   
   
   app = falcon.API(middleware=[
       SQLAlchemySessionManager(Session),
   ])
   

4. Session is accessible in every request.

   import falcon
   
   
   class MyAPI:
   
       def on_get(self, req, resp):
           # You can access self.session here
           # self.session.add(foo)
           # self.session.commit()
   

Rann

This was really helpful. One quick question, should that be resource.session.close() or resource.session.remove()? – Rarity

Neither, it should be Session.remove() according to docs.sqlalchemy.org/en/rel_1_1/orm/…. – Teratogenic

Then should it not be self.Session.remove()? – Sauger

How do you compare to eshlox.net/2019/05/28/… ? Is there any rollback support integrated into remove method ? Edit: the close method seem to be using rollback under the hood docs.sqlalchemy.org/en/13/orm/contextual.html – Agata

Answer 2

There is a package on pypi, falcon-sqla, that provides a middleware to manage SQLAlchemy sessions with Falcon.

It uses the request context object to add a different session to each http request, avoiding the need to use a scoped session.

Suction