Issues with scoped_session in sqlalchemy - how does it work?

Question

I'm not really sure how scoped_session works, other than it seems to be a wrapper that hides several real sessions, keeping them separate for different requests. Does it do this with thread locals?

Anyway the trouble is as follows:

S = elixir.session # = scoped_session(...)
f = Foo(bar=1)
S.add(f) # ERROR, f is already attached to session (different session)

Not sure how f ended up in a different session, I've not had problems with that before. Elsewhere I have code that looks just like that, but actually works. As you can imagine I find that very confusing.

I just don't know anything here, f seems to be magically added to a session in the constructor, but I don't seem to have any references to the session it uses. Why would it end up in a different session? How can I get it to end up in the right session? How does this scoped_session thing work anyway? It just seems to work sometimes, and other times it just doesn't.

I'm definitely very confused.

Answer 1

It turns out elixir sets save-on-init=True on the created mappers. This can be disabled by:

using_mapper_options(save_on_init=False)

This solves the problem. Kudos to stepz on #sqlalchemy for figuring out what was going on immediately. Although I am still curious how scoped_session really works, so if someone answers that, they'll get credit for answering the question.

Answer 2

Scoped session creates a proxy object that keeps a registry of (by default) per thread session objects created on demand from the passed session factory. When you access a session method such as ScopedSession.add it finds the session corresponding to the current thread and returns the add method bound to that session. The active session can be removed using the ScopedSession.remove() method.

ScopedSession has a few convenience methods, one is query_property that creates a property that returns a query object bound to the scoped session it was created on and the class it was accessed. The other is ScopedSession.mapper that adds a default __init__(**kwargs) constructor and by default adds created objects to the scoped session the mapper was created off. This behavior can be controlled by the save_on_init keyword argument to the mapper. ScopedSession.mapper is deprecated because of exactly the problem that is in the question. This is one case where the Python "explicit is better than implicit" philosophy really applies. Unfortunately Elixir still by default uses ScopedSession.mapper.