Add mix task to undo latest codegen/delete latest migration

[sevenseacat]

Is your feature request related to a problem? Please describe.

Codegen to generate a migration is really awesome, but sometimes you generate a migration and then you realize you missed something, so you want to delete that migration, make the change, and then generate it again. Currently that's a manual process - go find the migration, find the snapshots for all the relevant tables, delete them all, and do it again. It's pretty annoying!

Describe the solution you'd like

A task like ash_postgres.delete_latest_migration that will delete the last migration, and any snapshots that were created at the same time. No option to delete other migrations or anything like that - only the last one.

And it should probably confirm the name of the migration just to make sure!

Describe alternatives you've considered

You could just generate another migration with the added changes, but snapshot files are big and ugly and if you haven't even run the first migration, being able to undo/regenerate it seems like the smarter approach.

[zachdaniel]

There is an example here of a script that does it based off of git status:

https://hexdocs.pm/ash_postgres/migrations-and-tasks.html#regenerating-migrations

[zachdaniel]

I don't think we can just do "only the last one" because it is actually possible for the migration generator to produce more than one migration (concurrent index generation must be done in its own migration w/o a ddl transaction).

[sevenseacat]

Oh interesting! Yeah if I don't feel like doing it manually I usually do it with git now - add, stash, drop - but that feels a bit weird.

Okay so maybe not the last one, but any with the timestamp of the last one? If more than one migration is generated, do they have the same timestamp?

[zachdaniel]

They do not, the timestamp differentiates them. However, I think the fix here is actually to support a --dev flag for migrations, which uses numbered migrations like Dev1 and Dev2 etc. Then, when migrations are checked with mix ash.codegen --check, we error if there are dev migrations. When you run mix ash.codegen name we rollback all dev migrations, delete all dev snapshots and regenerate them fully.

The added benefit here (which is a big one) is we could then get:

1. a watcher that auto generates migrations with --dev as you change your files
2. a plug like Ecto's plug that says "you haven't run codegen yet, click here to run it"

[zachdaniel]

and then mix ash.codegen --reset-dev or something would then do what you want, based on dev migrations, not named migrations. Then you just name it once at the end when you're done with your feature.

[ken-kost]

a watcher that auto generates migrations with --dev as you change your files
a plug like Ecto's plug that says "you haven't run codegen yet, click here to run it"

could you expand on these two points, what you had in mind exactly. This sounds like the fun part. 👀

[zachdaniel]

https://hexdocs.pm/phoenix/Phoenix.Endpoint.html#module-runtime-configuration

So we would want something that can be plugged in here that would then autogenerate dev migrations on save with mix ash.codegen --dev

Then a plug like this one: https://hexdocs.pm/phoenix_ecto/Phoenix.Ecto.CheckRepoStatus.html

That runs mix ash.codegen --check and shows you a button to run mix ash.codegen --dev

[zachdaniel]

And I agree, these are definitely the fun part because they will make this --dev feature really sing and be a huge UX improvement.

[ken-kost]

watchers: [dev: {Mix.Tasks.AshPostgres.GenerateMigrations, :run, [["--dev"]]}],

works, i.e. runs dev migrations on server start. 🎉

Regarding the plug, where should that go, ash_phoenix?

Also, after we finish here I'll open a PR for codegen part so that will be in watcher configuration instead of generate migrations I assume, and docs should be expanded mentioning this possible configuration.

[zachdaniel]

Wow. That's really all that's needed? Does it run when files change too?

[zachdaniel]

And yeah the plug would go in ash_phoenix

[ken-kost]

Wow. That's really all that's needed? Does it run when files change too?

yea I also thought it would but I digged and it turns out it's misleading, the name watcher. 😅

https://elixirforum.com/t/phoenix-asset-management-watchers/45661/2?u=ken-kost