[Fixed]-How to use a model in a Python Django project for multiple applications?

If the linked model is in another app, you need to refer to that in the destination string.

exch_usr = models.ForeignKey('auth.User', on_delete=models.PROTECT)

Note, you do not need to import User at all.

If you are already importing User model on that models.py, you can just set it to ForeignKey. be aware that it’s not a string.

exch_usr = models.ForeignKey(User, on_delete=models.PROTECT)

The @danielr ‘s answer is perfect but I have a recommendation for you @Parry. So if the User model might be swapped out for another(thinking in the future) and to make your application as reusable as possible you should use the settings.AUTH_USER_MODEL as a references in your models . More information on customizing and referencing the User model can be found at https://docs.djangoproject.com/en/1.10/topics/auth/customizing/#referencing-the-user-model. And inside your code you must use the get_user_model Django(to create User references ) utility to create this switch in a clean way.

class SomeClass(models.Model):
....
custome_user = models.ForeignKey(settings.AUTH_USER_MODEL)

and to make references to User properties:

from django.contrib.auth import get_user_model
User = get_user_model()
class AnotherClass(models.Model):
....
username = User.USERNAME_FIELD