SQLAlchemy scoped_session is not getting latest data from DB - python-3.x

Question

I'm rather new to the whole ORM topic, and I've already searched forums and docs.
The question is about a flask application with SQLAlchemy as ORM for the PostgreSQL.
The __init__.py contains the following line:
db = SQLAlchemy()
the created object is referenced in the other files to access the DB.
There is a save function for the model:
def save(self):
db.session.add(self)
db.session.commit()
and also an update function:
def update(self):
for var_name in self.__dict__.keys():
if var_name is not ('_sa_instance_state' or 'id' or 'foreign_id'):
# Workaround for JSON update problem
flag_modified(self, var_name)
db.session.merge(self)
db.session.commit()
The problem occurs when I'm trying to save a new object. The save function writes it to DB, it's visible when querying the DB directly (psql, etc.), but a following ORM query like:
model_list = db.session.query(MyModel).filter(MyModel.foreign_id == this_id).all()
gives an empty response.
A call of the update function does work as expected, new data is visible when requesting with the ORM.
I'm always using the same session object for example this:
<sqlalchemy.orm.scoping.scoped_session object at 0x7f0cff68fda0>
If the application is restarted everything works fine until a new object was created and tried to get with the ORM.
An unhandsome workaround is using raw SQL like:
model_list = db.session.execute('SELECT * FROM models_table WHERE
foreign_id = ' + str(this_id))
which gives a ResultProxy with latest data like this:
<sqlalchemy.engine.result.ResultProxy object at 0x7f0cf74d0390>
I think my problem is a misunderstanding of the session. Can anyone help me?

Answer 1

It figured out that the problem has nothing to do with the session, but the filter() method:
# Neccessary import for string input into filter() function
from sqlalchemy import text
# Solution or workaround
model_list = db.session.query(MyModel).filter(text('foreign_key = ' + str(this_id))).all()
I could not figure out the problem with:
filter(MyModel.foreign_id == this_id) but that's another problem.
I think this way is better than executing raw SQL.