Howto

This is mainly for me to remember how to do things 😁.

Database Changes

Migration with Restore from Backup

Sometimes it’s not possible to do database changes via a Django migration. For example if you try to split up a model inheriting from Wagtails page model, it’s not possible to add / remove pages via a Django Migration because you don’t have access to the Page model in a migration (only the database).

Atm the best option for me is to copy the production database locally, do the migration in a notebook and then backup the migrated database and restore it in production. A manual migration is only needed for a database where there are models which should be added to the new model.

Steps

1. Backup old production database

   1. Fetch production database and restore it to the local development database

   2. Set site to localhost in wagtailadmin

2. Migrate the database structure

   1. Add a new model inheriting from the old one and prefix the attributes you want to keep with new_

   2. Create a new migration

   3. Use flit install -s to install the django-cast. package in the venv of your application

   4. Migrate

3. Migrate the database data manually

   1. Use a jupyter notebook to copy the old models over to the new model [example]

   2. Make sure to prefix uniqe page fields like slug with new first and rename it afterwards

   3. Remove the moved attributes from the old model

   4. Rename the attributes prefixed with new_ in the new model

4. Dump local database and restore to production

   1. Change site back to python-podcast.staging.wersdoerfer.de with port 443

   2. pg_dump python_podcast | gzip > backups/db.staging.psql.gz

   3. cd deploy && ansible-playbook restore_database.yml –limit staging

[example]

blog_to_podcast example

def blog_to_podcast(blog, content_type):
    exclude = {"id", "page_ptr_id", "page_ptr", "translation_key"}
    kwargs = {
        f.name: getattr(blog, f.name)
        for f in Blog._meta.fields
        if f.name not in exclude
    }
    kwargs["slug"] = f"new_{blog.slug}"
    kwargs["content_type"] = content_type
    kwargs["new_itunes_artwork"] = blog.itunes_artwork
    kwargs["new_itunes_categories"] = blog.itunes_categories
    kwargs["new_keywords"] = blog.keywords
    kwargs["new_explicit"] = blog.explicit
    return Podcast(**kwargs)

# first migration to add podcast model
from django.core.management import call_command
call_command("migrate")

# get the original blog + parent
original_slug = "show"
blog = Blog.objects.get(slug=original_slug)
blog_parent = Page.objects.parent_of(blog).first()

# fix hostname and port
site = Site.objects.first()
site.hostname = "localhost"
site.port = 8000
site.save()

# create new page
podcast_content_type = ContentType.objects.get(app_label="cast", model="podcast")
podcast = blog_to_podcast(blog, podcast_content_type)
podcast = blog_parent.add_child(instance=podcast)

# fix treebeard, dunno why this is needed
from django.core.management import call_command
call_command("fixtree")
podcast = Podcast.objects.get(slug=f"new_{origninal_slug}")  # super important!

# move children - this is extremely brittle!
from wagtail.actions.move_page import MovePageAction
for child in blog.get_children():
    mpa = MovePageAction(child, podcast, pos="last-child")
    mpa.execute()

# delete old page
blog.delete()

# restore slug
podcast.slug = original_slug
podcast.save()