[Sqlalchemy-tickets] Issue #3977: Using "configure" on scoped_session does not bind engine properly (zzzeek/sqlalchemy)

[Adam O. <iss...@bi...>]

New issue 3977: Using "configure" on scoped_session does not bind engine properly
https://bitbucket.org/zzzeek/sqlalchemy/issues/3977/using-configure-on-scoped_session-does-not

Adam Olsen:

Using SQLAlchemy 1.1.9 with Python 2.7.13 on OS X

Using `scoped_session.configure` to change the engine doesn't bind the engine properly.  For instance, if you do something like this:

```
#!python

session = orm.scoped_session(orm.sessionmaker())
engine = create_engine('sqlite://')
session.configure(bind=engine)
session.close_all()

engine = create_engine('postgresql://someuser:somepass@localhost:5432/db',
    json_serializer=serialize, json_deserializer=deserialize)
session.configure(bind=engine)

print session.bind  # will output `Engine(sqlite://)`
```

It results in errors such as the one you see below (if you start with sqlite, but then switch to postgres and try to query tables).

If you're wondering WHY we would want to do this, it is for testing.  Our test suite tries to make sure that everything works ok with both sqlite and postgresql.  We can't just create a new instance of the scoped_session, as it will break the factories and various other places that call for a session at load time.

```
#!python

Traceback (most recent call last):
  File "/vagrant/apps/skedup/tests/base.py", line 28, in setUp
    db.session.commit()
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/scoping.py", line 157, in do
    return getattr(self.registry(), name)(*args, **kwargs)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 874, in commit
    self.transaction.commit()
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 459, in commit
    self._assert_active(prepared_ok=True)
  File "/usr/local/lib/python2.7/site-packages/sqlalchemy/orm/session.py", line 276, in _assert_active
    % self._rollback_exception
InvalidRequestError: This Session's transaction has been rolled back due to a previous exception during flush. To begin a new transaction with this Session, first issue Session.rollback(). Original exception was: (exceptions.AttributeError) 'SQLiteDialect_pysqlite' object has no attribute '_json_serializer'
```

Doing something like this totally fixes it (manually assigning the engine to `session.bind`), but it seems like it shouldn't be required:

```
#!python

session = orm.scoped_session(orm.sessionmaker())
engine = create_engine('sqlite://')
session.configure(bind=engine)
session.close_all()

engine = create_engine('postgresql://someuser:somepass@localhost:5432/db',
    json_serializer=serialize, json_deserializer=deserialize)
session.configure(bind=engine)
session.bind = engine  # <---- this is the key to making it work

print session.bind  # will output `Engine(postgresql://someuser:somepass@localhost:5432/db)`
```