Data migration of image model

Question

Hi this is mostly a copy paste of a question asked in Google groups:

Thanks to Wagtail docs, I was able to understand how to build a custom image model, BUT, as I have a website with more than 500 contents, I don't want to mess the whole thing up with a bad data migration. In fact, I am not sure of which migration operation I should use here. I think that I should this one: https://docs.djangoproject.com/en/1.8/ref/migration-operations/#altermodeltable Can someone confirm this ?

Thanks a lot Regards

EDIT: provide some details about the migration

• I created a custom image model in order to provide image URLs directly into Wagtail API, on the image endpoint (example: https://github.com/wagtail/wagtaildemo/blob/api-tweaks/demo/models.py#L784-L795)
• As it is explained in wagtail docs, a data migration is required. But as I never been through this procedure, I just want to be sure to do it the right way

This is surely wagtail-specific, why I may have omitted some details. Sorry for that. And as I don't have much reputation, I can't provide more than 2 links a single post 😞

Answer 1

I recently did this same migration to my own customised model, however, we were not actively using Tags so I did not worry about transferring tags across. This migration does not delete the original Image database records as I wanted to keep them just in case.

Step 1 - Create Model app_name/models.py

from django.db import models
from wagtail.wagtailimages.models import (
    Image, AbstractImage, AbstractRendition)


class ExtendedImage(AbstractImage):
    caption = models.CharField(max_length=255, blank=True)

    admin_form_fields = Image.admin_form_fields + (
        'caption',
    )


class ExtendedRendition(AbstractRendition):
    image = models.ForeignKey(ExtendedImage, related_name='renditions')

    class Meta:
        unique_together = (
            ('image', 'filter_spec', 'focal_point_key'),
        )

Step 2 - Run Migrations Looks like you may have done this, it creates the customised model

1. $python manage.py makemigrations
2. $python manage.py migrate

Step 3 - Create Custom Data Migration

1. $python manage.py makemigrations --empty app_name

2. Edit that file as below (see comments inline)

```

from __future__ import unicode_literals

from django.db import migrations

# This only COPIES images from the existing model to the new one
# to reverse during testing - run
# ./manage.py migrate main 0036_auto_20170524_1811 (replace with file name of previous migration)


def forwards_func(apps, schema_editor):
    # We get the model from the versioned app registry;
    wagtail_image_model = apps.get_model('wagtailimages', 'Image')
    extended_image_model = apps.get_model('main', 'ExtendedImage')
    db_alias = schema_editor.connection.alias
    # Get images
    images = wagtail_image_model.objects.using(db_alias).all()
    new_images = []
    for image in images:
        new_images.append(extended_image_model(
            id=image.id,
            title=image.title,
            file=image.file,
            width=image.width,
            height=image.height,
            created_at=image.created_at,
            focal_point_x=image.focal_point_x,
            focal_point_y=image.focal_point_y,
            focal_point_width=image.focal_point_width,
            focal_point_height=image.focal_point_height,
            file_size=image.file_size,
            # image=test_image.caption,
            collection=image.collection,
            # tags=image.tags, # does not copy over
            uploaded_by_user=image.uploaded_by_user,
        ))
    # Create images in new model
    extended_image_model.objects.using(db_alias).bulk_create(new_images)
    # Leave all images in previous model


def reverse_func(apps, schema_editor):
    # We get the model from the versioned app registry;
    extended_image_model = apps.get_model('main', 'ExtendedImage')
    db_alias = schema_editor.connection.alias
    # Delete all images created in the new model
    extended_image_model.objects.using(db_alias).all().delete()


class Migration(migrations.Migration):

    dependencies = [
        ('main', '0036_auto_20170524_1811'), # Django will create this part
    ]

    operations = [
        migrations.RunPython(forwards_func, reverse_func),
    ]

```

Step 4 - Update Settings

WAGTAILIMAGES_IMAGE_MODEL = 'my_app.ExtendedImage'

Test this along the way and when you are ready you can delete the original image database rows if want.

** Note about Postgres One issue we ran into was Postgres did not like me migrating things to the primary key, we had to run a SQL query to reset the current key to the max + 1