We often need to link data to the user model used by Django, for example to store the author of a Post. People often refer to the User model [Django-doc] directly, for example with:

from django.db import models
from django.contrib.auth.models import User

class Post(models.Model):
    author = models.ForeignKey(
        User,
        on_delete=models.CASCADE
    )

Why is it a problem?

One does not per se needs to use Django's User model. It is possible that one now sticks to Django's default User model, but later one defines a custom user model, then one has to change all ForeignKeys, which is cumbersome, and error-prone.

One can make use of the get_user_model() [Django-doc] to obtain a reference to the class of the user model, but this is still not ideal: it requires to load the user model before we load the application where we reference that model. This thus makes the project less flexible.

What can be done to resolve the problem?

We can make use of the AUTH_USER_MODEL setting [Django-doc]. This is a string setting that contains the app_name.ModelName of the user model that is in use. Django thus will thus construct a ForeignKey with a string as target. This target is then, when all apps are loaded, resolved to the corresponding model.

We thus can thus let the ForeignKey reference the value of the AUTH_USER_MODEL setting:

from django.conf import settings
from django.db import models

class Post(models.Model):
    author = models.ForeignKey(
        settings.AUTH_USER_MODEL,
        on_delete=models.CASCADE
    )