from sqlalchemy_sessionload import SQLAlchemySessionLoad

Session = sessionmaker(...)
SQLAlchemySessionLoad(Session)

from sqlalchemy_sessionload import SessionLoad
from project.model import Message

# assignment is needed
# otherwise instances are not saved in session
all_messages = session.query(Message).all()

session_messages = session.query(Message).options(SessionLoad(Message)).all()

from sqlalchemy_sessionload import SessionRelationshipLoad
from project.model import Message, User
import sqlalchemy.orm as sa_orm

# assignment is needed
# otherwise instances are not saved in session
all_users = session.query(User).all()


# users connected to messages are now loaded from session
session_messages = (
    session.query(Message)
    .options(sa_orm.subqueryload(Message.user), SessionRelationshipLoad(Message.user))
    .all()
)