def find_meter_by_serial_number(meter_serial_number)
      unless @meters_by_serial_number.key?(meter_serial_number)
        meters = Meter.where(meter_serial_number: meter_serial_number)
        raise DataFeedException.new("Multiple meters found with meter_serial_number #{meter_serial_number}") if meters.size > 1
        @meters_by_serial_number[meter_serial_number] = meters.first