Why would I ever want to revert a migration?

Question

In Rails, migrations have a down method by default for reverting a migration. In what scenario would I ever want to revert a migration, though?

Some thoughts:

Whether in development or production, I always have a snapshot of my database to go back to, before I even run the migrations. Especially for migrations which perform data conversion, I find in most cases that reverting a snapshot is even faster than reverting a migration. (So I would never do it in a rush!)

If a migration were to fail, it would either:

• fail with an exception on a non-transactional database, and thus leave the database broken, or
• fail with an exception and roll back the transaction, and thus there would be no need to revert otherwise.

If the changes made are in production (or late in development), and later turn out to be a mistake, I would fix my mistake in a new migration. I would not revert the old one. In development, I'd simply delete the migration.

I also find that the down method introduces extra code in which I repeat myself, and thus may introduce new bugs. This is against the DRY principle.

So I'm curious about the pros, because I can't think of any.

Answer 1

In development, it is easy and fast to incrementally "improve" migrations by using the down method automatically. Eg

1. Create a migration and migrate to it
2. Realize you need to make a change
3. Migrate to the ver prior to your new migration by using db:migrate with a version
4. Improve/fix your migration
5. Rerun the migration task

Your method of taking snapshots works fine. But rails includes the same effect auto-magically using the "down" migration techniques. Works with all db's, tastes great

Added:

For production, I agree that a down migration shouldn't be needed. But sometimes mistakes happen and you need to roll back. The down migration path gives you a first, and quick opportunity to fix things in an emergency situation during an upgrade that goes wrong.

-- it is much faster to try a down migration in an emergency than to restore the db using a checkpoint.