Dear Michael,
I have discovered that under very particular circumstances, SQLAlchemy forgets
that an object was newly created in a transaction, and if that transaction
rolls back, the object's state is not reset from persistent to transient.
It seems to rely on the following:
* Open an outer transaction
* Open an inner transaction, add object to session, close transaction to
commit. The object is now persistent.
* Clear all references to the object, allowing it to be GC'ed and
removed from the session._new weakref map
* Load the object from the database again.
* Raise an exception to rollback the outer transaction.
* SQLAlchemy does not realise that the object is new, and fails to reset
its state.
* Object is still "persistent", but no longer in the database.
Here is an example that reproduces the problem:
from sqlalchemy import Column, Integer, Text, create_engine, inspect
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
Base = declarative_base()
class Dog(Base):
__tablename__ = 'dog'
id = Column(Integer, primary_key=True)
name = Column(Text)
def create():
global session
with session.begin(nested=True):
fido = Dog(name="fido")
session.add(fido)
assert not inspect(fido).persistent, "should not yet be persisted to
database"
assert inspect(fido).persistent, "should now be persisted to database, but
transaction not yet committed"
def fetch():
global session
global fido
fido = session.query(Dog).filter_by(name="fido").one()
assert inspect(fido).persistent, "should have been retrieved from the
database, therefore persistent"
def main():
engine = create_engine('sqlite://')
engine.echo = True
Base.metadata.create_all(engine)
DBSession = sessionmaker(bind=engine)
global session
session = DBSession(autocommit=True)
try:
with session.begin(subtransactions=True) as trans:
create()
print(dict(trans._new))
fetch()
print(dict(trans._new))
raise Exception("force transaction to roll back")
except Exception:
pass
global fido
assert not inspect(fido).persistent, "after rollback, should no longer be
in the database"
if __name__ == '__main__':
main()
Which fails the last assertion:
AssertionError: after rollback, should no longer be in the database
I know that this is a minor issue, but I found it quite confusing while
debugging.
I suspect that we might need to keep a strongly referenced list of "newly
added" PKs in the outer session, instead of using _new.
Thanks, Chris.
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