class User(DB.Model):
    __tablename__ = 'users'
    id = DB.Column(DB.Integer, primary_key=True)
    username = DB.Column(DB.String(64), index=True)
    password = DB.Column(DB.String(128))

class Customer(DB.Model):
    __tablename__ = 'customer'
    customer_id = DB.Column(DB.Integer, primary_key=True)
    customer_name = DB.Column(DB.String, unique=True, index=True)

SQLALCHEMY_BINDS = {
    'customer1' = 'sqlite:////path/customer1.db',
    'customer2' = 'sqlite:////path/customer2.db',
    ...
}

def get_session(customer_id):
    sqlite_url = 'sqlite:////path/customer%s.db' % customer_id
    engine = create_engine(sqlite_url)

    # initialize the db if it hasn't yet been initialized
    Base.metadata.create_all(engine)

    Session = sessionmaker(bind=engine)
    session = Session()

    return session