[Sqlalchemy-tickets] Issue #3980: TypeError: issubclass() arg 1 must be a class (zzzeek/sqlalchemy)

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

New issue 3980: TypeError: issubclass() arg 1 must be a class
https://bitbucket.org/zzzeek/sqlalchemy/issues/3980/typeerror-issubclass-arg-1-must-be-a-class

Anthony Oduor:

In implementing a multi-tenant system using PostreSQL schemas... When A new customer signs-up I create a new schema when they activate their account. Here is the code for the function that creates the schema and even put's in a dummy record.


```
#!python
settings = request.registry.settings
engine = get_engine(settings)

Session = sessionmaker(engine, expire_on_commit=False)
Session.configure(bind=engine)
session = Session()

session.begin(subtransactions=True)
try:
    engine.execute(CreateSchema(tenant_id))

    metadata = MetaData(schema=tenant_id)
    declared_base = declarative_base(bind=engine, name='NewTenantBase', metadata=metadata)

    NewTenantBase = automap_base(declarative_base=declared_base)
    NewTenantBase.metadata.schema = tenant_id
    # NewTenantBase.metadata.reflect(bind=engine)

    NewTenantBase.prepare(engine, reflect=False)
    configure_mappers()

    class MyBooks(BooksMixin, NewTenantBase):
        __tablename__ = 'mybooks'

    # mapped classes are ready
    MyBooks = NewTenantBase.classes.mybooks

    NewTenantBase.metadata.create_all()

    try:
        # set the search path
        session.execute("SET search_path TO %s" % tenant_id)

        session.add(MyBooks(name='Fiction', description='You know what fiction are'))
        session.flush()
        session.execute("commit")
    except Exception as e:
        traceback.print_exc()
        session.rollback()

    session.commit()
except Exception as e:
    traceback.print_exc()
    session.rollback()
```

This using Pyramid-Websauna framework. The first signup goes well and everything is setup properly in the new schema without an error. The problem is subsequent signups don't go as well. I get the error below which happens after the new schema has been created. 

```
#!python

TypeError('issubclass() arg 1 must be a class',)
Traceback (most recent call last):
  File "/opt/mapylons/myproject/myapp/myapp/tasks.py", line 80, in create_schema
    NewTenantBase.prepare(engine, reflect=False)
  File "/opt/mapylons/myproject/venv3.5/lib/python3.5/site-packages/sqlalchemy/ext/automap.py", line 787, in prepare
    generate_relationship)
  File "/opt/mapylons/myproject/venv3.5/lib/python3.5/site-packages/sqlalchemy/ext/automap.py", line 894, in _relationships_for_fks
    local_cls, referred_cls):
TypeError: issubclass() arg 1 must be a class
```

The interesting bit is that is when I restart the server between each signup, they will all go well and the customer schema setup will go on without a hitch. How can I make this multi-tenancy work properly without literally restarting `pserve` after each customer signup? ...which would be quite a hack job