Update Wagtail's image model on an existing project

migration.py

# Generated by Django 2.2.24 on 2021-08-17 11:33

from django.db import migrations
from django.forms.models import model_to_dict
from wagtail.images.models import Image as PreviousImage

from image.models import CustomImage as Image

def migrate_images(apps, schema_editor):
    for image in PreviousImage.objects.all():
        new_image_data = {}
        
        for field in PreviousImage._meta.fields:
            new_image_data[field.name] = getattr(image, field.name)

        new_image = Image(**new_image_data)
        new_image.id = image.id
        new_image.save()
        

def remove_images(apps, schema_editor):
    for image in Image.objects.all():
        image.delete()

class Migration(migrations.Migration):

    dependencies = [
        ("image", "0001_initial"),
    ]

    operations = [
        migrations.RunPython(migrate_images, reverse_code=remove_images),
    ]

If ever you need to add a custom wagtail image model, you might have some issues migrating your previously added images to your brand new model.

Here is a little gist to help you do such a thing.

Add a new image model

Start by adding your image model, once it is done, create an empty migration python manage.py makemigrations --empty yourappname (https://docs.djangoproject.com/fr/3.2/topics/migrations/)

Write the migration

Adapt the content of your empty migration to match what is written in this gist.

Deploy your changes

If you have an app running on production, you should start by deploying these changes without changing the default Wagtail's image model.

Switch Wagtail's image model

Once your changes are live, your migrations properly ran, you can safely switch the image model in the wagtail settings/base.py file