Django - migrating to a custom user model mid-project #

This was quite painful. Please just follow the best practice of using a custom user model from the start.

Starting point in my app #

Default user model
One model proxying the current user model, because I didn’t want to do this change before
Wanted to add a new app to contain the new custom user model (shared)
Using django-tenants

Process #

First stage #

Create a new app, shared, to contain the new user model User. Just with django-admin startapp shared. Clean up files that won’t be used like views.py etc.
Update settings.py: Add this app to all tenants
Before making any changes to models: Create a single, blank migration for this app, i.e. 0001_initial.py that does nothing (python manage.py makemigrations --empty shared)
Run python manage.py migrate to get this first initial app set up.

Here, between the stages, you should push these changes and run the migrations against your production and any other DBs that you wish to persists.

Second stage #

Now update the models.py in shared app with your new model, like the one below. Do not make any other changes at this point preferably, if you want to add new fields, do it later once the change process is complete.
```
from django.db import models
from django.contrib.auth.models import AbstractUser

class User(AbstractUser):
    class Meta:
        db_table = 'auth_user'
```
Edit the shared/migrations/0001_initial.py migration that you had created before to handle the creation of the entire User model. Quoting below from one of the references on what to do:
1. This is because you want the initial migration to already have been executed in an instance with data (hence the first stage), but in the future you’ll want to recreate it completely in new environments (e.g. in testing)
2. (The exact code to use here will likely change over time with newer versions of Django. You can find the current code by creating a new app temporarily, add the User model to it and then look at the migration file ./manage.py makemigrations produces.)
Update the Django setting to use your new model AUTH_USER_MODEL = "shared.User"
Create a new migration with manage.py makemigrations --empty shared

Edit that second migration, called something like shared/migrations/0002_xx.py, adding a function that will convert the user model entries from the old model to the new one:

def change_user_type(apps, schema_editor):
    ContentType = apps.get_model('contenttypes', 'ContentType')
    ct = ContentType.objects.filter(
        app_label='auth',
        model='user'
    ).first()
    if ct:
        ct.app_label = 'user'
        ct.save()

Now run your second migration with manage.py migrate
All done! Double check everything, including logging in and the various pages of your app.

Additional issues I encountered #

Wrong default manager in the new User model didn’t allow me to log in, due to previously screwing up with the managers of my proxy model. (default query was returning active, non-staff users only, hence my superuser logins were not working)
Postgres versions in different envs. dbbackup needs a matching Postgres version in order to function correctly.
Previous references to ActiveTenantUser via a relationship from VotingEvent needed to be manually changed. The table column name of activetenantuser_id wouldn’t change, so needed to do this manually in postgres in Prod