Using SQLAlchemy sessions with flask and concurrency issues

I'm developing an API with Flask and SQLAlchemy and here's what I want to do:

I have a client application that works on multiple tablets and has to send several requests to add content to the server. But I don't want to use automatic rollback at the end of every API request (default behavior of flask-sqlalchemy), because the sending of data is done over multiple requests, like in this very simplified example:

1. beginTransaction /? id = transactionId - > open a new session for the client making that request. SessionManager.new_session() in the following code .

2. addObject /? id=objectAid - > add an object to the PostGreSQL database and refresh

3. addObject /? id=objectBid - > add an object to the PostGreSQL database and refresh

4. commitTransaction /? id = transactionId - > Commits what happened since beginTransaction. SessionManager.commit() in the following code .

The point here is that if the client app crashes/loses the connection before sending the "commitTransaction", the data is not added to the server, preventing incomplete data on the server.

Since I don't want to use automatic rollback, I can't really use flask-SQLAlchemy, so I'm implementing SQLAlchemy myself into my flask app, but I'm not sure how to use sessions.

Here is my implementation in __init__.py:

db = create_engine('postgresql+psycopg2://admin:pwd@localhost/postgresqlddb',
                   pool_reset_on_return=False,
                   echo=True, pool_size=20, max_overflow=5)

Base = declarative_base()
metadata = Base.metadata
metadata.bind = db

# create a configured "Session" class
Session = scoped_session(sessionmaker(bind=db, autoflush=False))



class SessionManager(object):

    currentSession = Session()

    @staticmethod
    def new_session():
    #if a session is already opened by the client, close it
    #create a new session
        try:
            SessionManager.currentSession.rollback()
            SessionManager.currentSession.close()
        except Exception, e:
            print(e)

        SessionManager.currentSession = Session()
        return SessionManager.currentSession

    @staticmethod
    def flush():
        try:
            SessionManager.currentSession.flush()
            return True
        except Exception, e:
            print(e)
            SessionManager.currentSession.rollback()
            return False

    @staticmethod
    def commit():
    #commit and close the session
    #create a new session in case the client makes a single request without using beginTransaction/
        try:
            SessionManager.currentSession.commit()
            SessionManager.currentSession.close()
            SessionManager.currentSession = Session()
            return True
        except Exception, e:
            print(e)
            SessionManager.currentSession.rollback()
            SessionManager.currentSession.close()
            SessionManager.currentSession = Session()
            return False

But now, when multiple clients make requests, the API doesn't work, it seems that each client is sharing the same session.

How should I implement sessions so that each client has a different session and can make requests at the same time?

Thank you.