Abstract model error

#1

hi,I have problem about the abstract model in chapter “Adding Easy Admin Emails, Helpers, Sitemaps, and More” (the hello web app intermediate book)
here is my models.py code:

when I run
$python manage.py makemigrations
$ python manage.py migrate
the error message shows:

/helloApp/venv/lib/python2.7/site-packages/django/db/models/base.py:309: RuntimeWarning: Model ‘collection.thing’ was already registered. Reloading models is not advised as it can lead to inconsistencies, most notably with related models.
new_class._meta.apps.register_model(new_class._meta.app_label, new_class)

No changes detected

that does not show like in the book :

You are trying to add a non-nullable field ‘added’ to thing without a defaul,

any answer would very greatful thank you!

#2

that seems I solve the problem from stackoverflow

#3

Good to know it was solved! What fixed it?

#4

I #out the class Thing(Timestamp):,. and change the class Thing(models.Model): ,.to class Thing(Timestamp) , it can run like this "You are trying to add a non-nullable field ‘added’ to thing without a defaul,.
I thought it has two class Thing(***),but it needs only one