instance method start

# File activerecord/lib/active_record/connection_adapters/abstract/transaction.rb, line 90
      def start
        raise InstrumentationAlreadyStartedError.new("Called start on an already started transaction") if @started
        @started = true

        ActiveSupport::Notifications.instrument("start_transaction.active_record", @base_payload)

        @payload = @base_payload.dup # We dup because the payload for a given event is mutated later to add the outcome.
        @handle = ActiveSupport::Notifications.instrumenter.build_handle("transaction.active_record", @payload)
        @handle.start
      end