Migrating models of dependencies when changing DEFAULT_AUTO_FIELD

Question

I'm using Django 3.2. I've changed added this line to settings.py:

DEFAULT_AUTO_FIELD = 'django.db.models.BigAutoField'

I then ran these commands:

$ python manage.py makemigrations
$ python manage.py migrate

The makemigrations command creates new migration files for my apps, not just the apps that I have created, but also in my dependencies. For example, I'm using django-allauth, and this file was created in my virtual environment (virtualenv):

 .venv/lib/python3.8/site-packages/allauth/account/migrations/0003_auto_20210408_1526.py

This file is not shipped with django-allauth. When I deploy this application from git, this file is not included.

What should I do instead? How can I switch DEFAULT_AUTO_FIELD without the need to create new migration files for dependencies like django-allauth?

Answer 1

I work on a big project, we upgraded Django from 2.2. to 3.2 and then have got a need to create all new models with Big Integer (Int8) (PostgreSQL) field instead of default Integer (Int4).

When I defined it in settings.py:

DEFAULT_AUTO_FIELD = 'django.db.models.BigAutoField'

I got the same problem, but with own apps - Django tried to make me to migrate 135 models I had, but I didn't want to do it. I only wanted to create new models with BigInt and manipuate olds manually.

I found the next solution. I changed the field to custom:

DEFAULT_AUTO_FIELD = 'project.db.models.CustomBigAutoField'

And then overrided its deconstruction:

from django.db import models

class CustomBigAutoField(models.BigAutoField):
    """Int8 field that is applied only for new models."""

    def deconstruct(self):
        name, path, args, kwargs = super().deconstruct()
        if getattr(self, 'model', None):
            path = 'django.db.models.AutoField'
        return name, path, args, kwargs

As I discovered, fields of new models don't have a back reference to their models, so path wouldn't be overridden for them. We override path because Django checks whether a model is changed by a key, that includes the path to this field. So we deceive Django and it thinks that existing model didn't changed.

I might not see the whole picture, but I tested it with different existing and new models and it worked for me. If someone tells me why this solution is bad, I'd be grateful.