til/rails/mark-a-migration-as-irreversible.md

Mark A Migration As Irreversible

It is in your best interest to, as much as is possible, write your Rails migrations in a way that they can be safely and reliably rolledback. You want your down to mirror your up, in case anything goes wrong.

This isn't always possible though. There are some migrations, in particular data migrations, that cannot be undone. Something is being changed or destroyed in an unrecoverable way. When this is the case, you should, by convention, raise an IrreversibleMigration exception.

class DestructiveMigration < ActiveRecord::Migration[5.2]
  def up
    execute "-- some destructive SQL"
  end

  def down
    raise ActiveRecord::IrreversibleMigration
  end
end

If anyone ever tries to rollback this migration, they will see the exception. It will be a signal that some manual work is needed to continue rolling back.

See the docs for more details.