pallets-eco
Fix init & multiple app issues with SQLAlchemy interface
These are some proposed fixes to issues I have run into with integrating Flask-Session into our application. The two problems were:
- the call to db.create_all() was causing problems as described here: https://github.com/fengsp/flask-session/issues/11
- When instantiating an application multiple times in the same process (in our case, during testing) the method used to create the SA table/model would fail b/c it already existed. But a check in for this so it's only created once for each DB.
@fengsp it would be very helpful to have this fixed. What can I do to get it moving?
@rsyring We need to call the create_all. Otherwise, how to do that for users? The session table detail is hidden. Any good ideas?
@fengsp my suggestion would be to create an entities.py file, and create a SessionMixin class which the user of library could include in their own SQLA metadata / entities however they want.
You might also provide an alembic script in the docs with a raw sql version if developers are trying to migrate.
Flask-Session would then, as part of the init_app be handed the entity which should be used to construct a session object.
The create_all call is not necessary, we deleted it in our branch because it was problematic.
Instead of the create_all() call you could use Session.__table__.create(bind=db.engine, checkfirst=True). That will create the table for the session model if it doesn't already exist.
moving to flask-session 0.3.1 from 0.3.0 made the flask_session table stop getting created when doing my initial alembic schema migration. something changed that broke our use case. how are we supposed to deploy the flask_session table now when doing an initial schema migration/deploy?
@revmischa You should do the db creation by yourself, including your other models.
The fix for recreating the session model is still very needed. I have to fork this repo in order to do tests, or else I get errors from the test suite as it tries to redefine this table on every initialization of the plugin
I am using 0.4.0, and am still having this problem:
-- I have around 396 pytest cases. For 3 ( three ) particular test cases, if I run only them together, two ( 2 ) would fail.
I came to this post,
https://stackoverflow.com/questions/64721498/how-do-you-resolve-already-defined-in-this-metadata-instance-error-with-flask How do you resolve 'Already defined in this MetaData Instance' Error with Flask Pytest, SqlAlchemy, and Flask Sessions?
I saw @tassaron answer. If I modified flask_session/sessions.py directly, the way @tassaron does:
class SqlAlchemySessionInterface(SessionInterface):
...
def __init__(self, app, db, table, key_prefix, use_signer=False,
permanent=True):
...
if table not in self.db.metadata:
class Session(self.db.Model):
...
# self.db.create_all()
self.sql_session_model = db.session_ext_session_model = Session
else:
self.sql_session_model = db.session_ext_session_model
My problems go away. We do need:
else:
self.sql_session_model = db.session_ext_session_model
Otherwise later on it would complain self.sql_session_model not exist.
Thank you and best regards.
...behai.
This is how I get around this problem. It feels like a hack though:
"""
Flask-Session version 0.4.0 -- latest as on 24/November/2022.
Work around / hack for getting around the exception during tests:
sqlalchemy.exc.InvalidRequestError: Table 'sessions' is already defined
for this MetaData instance. Specify 'extend_existing=True' to redefine
options and columns on an existing Table object.
Usage:
try:
from book_keeping.library.fixed_session import FixedSession as Session
except ImportError:
from flask_session import Session
References:
https://stackoverflow.com/questions/64721498/how-do-you-resolve-already-defined-in-this-metadata-instance-error-with-flask
https://github.com/fengsp/flask-session/blob/1c1f7903184673682bd1d75432c8f455b62393a4/flask_session/sessions.py
https://github.com/tassaron/muffin-shop/blob/main/src/helpers/main/session_interface.py
https://github.com/fengsp/flask-session/pull/12
"""
from flask_session.sessions import SqlAlchemySessionInterface
from flask_session import Session
class FixedSqlAlchemySessionInterface( SqlAlchemySessionInterface ):
def __init__(self, app, db, table, key_prefix, use_signer=False,
permanent=True):
"""
Assumption: the way I use it, db is always a valid instance
at this point.
"""
if table not in db.metadata:
super().__init__( app, db, table, key_prefix, use_signer, permanent )
db.session_ext_session_model = self.sql_session_model
else:
# print( "`sessions` table already exists..." )
self.db = db
self.key_prefix = key_prefix
self.use_signer = use_signer
self.permanent = permanent
self.has_same_site_capability = hasattr(self, "get_cookie_samesite")
self.sql_session_model = db.session_ext_session_model
class FixedSession( Session ):
def _get_interface(self, app):
config = app.config.copy()
if config[ 'SESSION_TYPE' ] != 'sqlalchemy':
return super()._get_interface( app )
else:
config.setdefault( 'SESSION_PERMANENT', True )
config.setdefault( 'SESSION_KEY_PREFIX', 'session:' )
return FixedSqlAlchemySessionInterface(
app, config['SESSION_SQLALCHEMY'],
config['SESSION_SQLALCHEMY_TABLE'],
config['SESSION_KEY_PREFIX'], config['SESSION_USE_SIGNER'],
config['SESSION_PERMANENT'] )
Hi @rsyring @tassaron @behai-nguyen this repo is under new maintainers. Would be grateful if you can test 0.6.0 and confirm if this fixed your issues.
Table option keep_existing has been used to fix this and prevent accidental schema changes from 0.6.0