Embed Uniqueness Issues on 2.12

Issue Summary

Embed uniqueness was not enforced at a db level pre-2.12, so it is possible for https://github.com/wagtail/wagtail/blob/master/wagtail/embeds/migrations/0008_allow_long_urls.py to fail during upgrade. (Additionally, because the uniqueness is enforced on the hash, rather than the url, there seems to be an additional issue with certain url strings that are apparently unique in postgres (a url_count annotation is 1 for both) but equal in Python, but I haven’t worked out exactly what the problem is here)

Steps to Reproduce

1. Start a new project with wagtail start myproject on Wagtail 2.11, making sure the embeds app is added
2. Enter the django shell
3. Create two Embeds objects with the same url using Embed.objects.create
4. Attempt the 2.12 upgrade

Technical details

• Python version: 3.6.4
• Django version: 2.2.13
• Wagtail version: 2.11.3 -> 2.12
• Browser version: N/A

[hanzvanaardt]

I ran into the same issue. It turns out that the issue is caused by a migration using pgloader (in my case this was done from SQLite to Postgres) but it could be from other databases as well. PGLoader converts constraints by creating them as indexes in PostgreSQL whereas with Django Migrations they are created as constraints. So during the migration the constraint is not found and an error is reported.
The issue can be resolved by following these steps: Enter the DB Shell ( python manage.py dbshell ) Step 1: DROP INDEX wagtailembeds_embed_url_max_width_8a2922d8_uniq; Step 2: ALTER TABLE public.wagtailembeds_embed ADD CONSTRAINT wagtailembeds_embed_url_max_width_8a2922d8_uniq UNIQUE(url, max_width);

If the above DROP INDEX does not work in Step 1 then execute \d wagtailembeds_embed to get the correct INDEX name.

[the-wong-dev]

I just resolved this same issue and the solution is pretty simple, especially if you have a GUI to interact with the database (I’m using HeidiSQL). For reference, I’m on Wagtail 2.13.1 and Django 3.2.4.

1. Drop the wagtailembeds_embed table.
2. Remove all ‘wagtailembeds’ migrations from the django_migrations table.
3. Run ‘python manage.py migrate’.

It’ll just rebuild the whole table cleanly.