How do I run a migration without starting a transaction in Rails?

There’s now a method disable_ddl_transaction! that allows this, e.g.:

class AddIndexesToTablesBasedOnUsage < ActiveRecord::Migration
  disable_ddl_transaction!
  def up
    execute %{
      CREATE INDEX CONCURRENTLY index_reservations_subscription_id ON reservations (subscription_id);
    }
  end
  def down
    execute %{DROP INDEX index_reservations_subscription_id}
  end
end