representation/printing of Models fails when model is not yet persisted to the database

[jeff-99]

When you create an new instance of a model you cannot print it or use it in logs, because the repr method of Model tries to loop through the identity parameter of inspect(), which is None.

After persisting to the database the instance can be printed.

example:

>>> from application.extensions import db
>>> from application.model.models import User
>>> u = User()
>>> print(u)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/home/jeff/virtualEnvs/kongoserver/lib/python3.5/site-packages/flask_sqlalchemy/model.py", line 121, in __repr__
    pk = ', '.join(to_str(value) for value in inspect(self).identity)
TypeError: 'NoneType' object is not iterable
>>> u.id = 12345
>>> print(u)
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/home/jeff/virtualEnvs/kongoserver/lib/python3.5/site-packages/flask_sqlalchemy/model.py", line 121, in __repr__
    pk = ', '.join(to_str(value) for value in inspect(self).identity)
TypeError: 'NoneType' object is not iterable
>>> db.session.add(u)
>>> db.session.commit()
>>> print(u)
<User 12345>

Where User is defined like:

class User(db.Model):
    __tablename__ = 'application_user'

    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(64), unique=True)
    password = db.Column(db.String(255))