def write(self, rootdir):
        """
        Write the metadata to a JSON file in the rootdir.

        Values contained in the metadata are:

        version : int
            The value of FORMAT_VERSION of this class.
        ohlc_ratio : int
            The default ratio by which to multiply the pricing data to
            convert the floats from floats to an integer to fit within
            the np.uint32. If ohlc_ratios_per_sid is None or does not
            contain a mapping for a given sid, this ratio is used.
        ohlc_ratios_per_sid : dict
             A dict mapping each sid in the output to the factor by
             which the pricing data is multiplied so that the float data
             can be stored as an integer.
        minutes_per_day : int
            The number of minutes per each period.
        calendar_name : str
            The name of the TradingCalendar on which the minute bars are
            based.
        start_session : datetime
            'YYYY-MM-DD' formatted representation of the first trading
            session in the data set.
        end_session : datetime
            'YYYY-MM-DD' formatted representation of the last trading
            session in the data set.

        Deprecated, but included for backwards compatibility:

        first_trading_day : string
            'YYYY-MM-DD' formatted representation of the first trading day
             available in the dataset.
        market_opens : list
            List of int64 values representing UTC market opens as
            minutes since epoch.
        market_closes : list
            List of int64 values representing UTC market closes as
            minutes since epoch.
        """

        calendar = self.calendar
        slicer = calendar.schedule.index.slice_indexer(
            self.start_session,
            self.end_session,
        )
        schedule = calendar.schedule[slicer]
        market_opens = schedule.market_open
        market_closes = schedule.market_close

        metadata = {
            'version': self.version,
            'ohlc_ratio': self.default_ohlc_ratio,
            'ohlc_ratios_per_sid': self.ohlc_ratios_per_sid,
            'minutes_per_day': self.minutes_per_day,
            'calendar_name': self.calendar.name,
            'start_session': str(self.start_session.date()),
            'end_session': str(self.end_session.date()),
            # Write these values for backwards compatibility
            'first_trading_day': str(self.start_session.date()),
            'market_opens': (
                market_opens.values.astype('datetime64[m]').
                astype(np.int64).tolist()),
            'market_closes': (
                market_closes.values.astype('datetime64[m]').
                astype(np.int64).tolist()),
        }
        with open(self.metadata_path(rootdir), 'w+') as fp:
            json.dump(metadata, fp)