flask-sqlalchemy bug

class SessionToken (db.Model):
  id = db.Column(db.Integer, primary_key=True)
  token = db.Column(db.String(64), unique=True)
  user = db.relationship("User", backref=db.backref("tokens", lazy="dynamic"))
  user_id = db.Column(db.Integer, db.ForeignKey("user.id"))
  expire_date = db.DateTime()
  def __init__ (self, user, validity_time=DEFAULT_VALIDITY_TIME):
    self.token=random_string()
    self.user=user
    self.expire_date=datetime.datetime.now()+validity_time
  def valid (self):
    return self.expire_date<datetime.datetime.now()