Rails migrations and the dangers of implicit commits

I recently came across the migration below. At first sight it looks like everything is okay, but there is actually a very dangerous assumption being made here.

# migration to convert table to utf8
class ConvertWidgetsTableToUtf8Unicode < ActiveRecord::Migration
  def up
    ActiveRecord::Base.transaction do
      table_name = 'widgets'
      say "converting #{table_name} table to utf8_unicode_ci"

      execute("ALTER TABLE #{table_name} CONVERT TO CHARACTER SET utf8 COLLATE utf8_unicode_ci")
      execute("ALTER TABLE #{table_name} DEFAULT CHARACTER SET utf8 COLLATE utf8_unicode_ci")

Notice how the utf8 conversion code is wrapped inside a transaction. The assumption here is that if something goes wrong the transaction will trigger a rollback. However, an ALTER TABLE command in MySQL causes an implicit commit. This means that the rollback will not undo any changes introduced by the ALTER TABLE command!