Python language
SQL Alchemy
Session handling by scoped_session
While using SqlAlchemy, remember how the sessions are managed by the sessionmaker.
scoped_session returns a registry
- It can be used directly or we can get an available session through it
- It handles creating a session if none is available
from sqlalchemy.orm import sessionmaker, scoped_session
sharded_session = sessionmaker(query_cls=CustomQuery, class_=ShardedSession)
session = scoped_session(sharded_session)
This session should be more like session_registry, althought its not
because you can use it directly, which has been misleading in the past.
For the sake of the example, let's assume it's called session_registry
You can either do:
session_registry.query(SomeClass)
or
session = session_registry()
session.query(SomeClass)
The end result is the same, but the first one hides the fact that it creates the session and keeps it available for further use within the registry
The real problem is understanding the lifecycle and when to clean up. The proposed solution in the docs is to just let the underlying API framework handling the removal of the session. Something like:
@on_request_end
def remove_session(req):
session_registry.remove()
But if you handle multiple DB connections, you can hide the fact that you need to clean the session before querying other DB.