I've been trying to map an object to a database using SQLAlchemy but have run into a snag.

Version info if handy: [OS: Mac OSX 10.5.8 | Python: 2.6.4 | SQLAlchemy: 0.5.8]

The class I'm going to map:

class Student(object):
    def __init__(self, name, id):
        self.id = id
        self.name = name
        self.preferences = collections.defaultdict(set)
        self.allocated_project = None
        self.allocated_rank = 0

    def __repr__(self):
        return str(self)

    def __str__(self):
        return "%s %s" %(self.id, self.name)

Background:

Now, I've got a function that reads in the necessary information from a text database into these objects. The function more or less works and I can easily access the information from the objects.

Before the SQLAlchemy code runs, the function will read in the necessary info and store it into the Class. There is a dictionary called students which stores this as such:

students = {}
students[id] = Student(<all the info from the various "reader" functions>)

Afterwards, there is an "allocation" algorithm that will allocate projects to student. It does that well enough. The allocated_project remains as None if a student is unsuccessful in getting a project.

SQLAlchemy bit:

So after all this happens, I'd like to map my object to a database table. Using the documentation, I've used the following code to only map certain bits. I also begin to create a Session.

from sqlalchemy import *
from sqlalchemy.orm import *

engine = create_engine('sqlite:///:memory:', echo=False)
metadata = MetaData()
students_table = Table('studs', metadata,
    Column('id', Integer, primary_key=True),
    Column('name', String)
)
metadata.create_all(engine)
mapper(Student, students_table)

Session = sessionmaker(bind=engine)
sesh = Session()

Now after that, I was curious to see if I could print out all the students from my students dictionary.

for student in students.itervalues():
    print student

What do I get but an error:

Traceback (most recent call last):
  File "~/FYP_Tests/FYP_Tests.py", line 140, in <module>
    print student
  File "/~FYP_Tests/Parties.py", line 30, in __str__
    return "%s %s" %(self.id, self.name)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/site-packages/SQLAlchemy-0.5.8-py2.6.egg/sqlalchemy/orm/attributes.py", line 158, in __get__
  return self.impl.get(instance_state(instance), instance_dict(instance))
AttributeError: 'NoneType' object has no attribute 'get'

I'm at a loss as to how to resolve this issue, if it is an issue. If more information is required, please ask and I will provide it.