models.py

# Omitting typical SQLAlchemy setup

class User(Base):
    __tablename__ = "users"
    id = Column(Integer, primary_key=True)
    username = Column(String(32))
    # ...

class Users(object):
    def __getitem(self, username):
        session = DBSession()
        try:
            return session.query(User).filter_by(username=username).one()
        except:
            raise KeyError, username

def get_root():
    return {"users": Users()}