code: def save_portfolio(self, datetime, portfolio):
          '''
          Store in Rethinkdb a zipline.Portfolio object
          '''
          log.debug('Saving portfolio in database')
          pf = dbutils.portfolio_to_dict(portfolio)
          pf.pop('positions')
          data = [{
              "name": self.name,
              "time": datetime.strftime("%Y-%m-%d %H:%M"),
              "columns": pf.keys(),
              "points": [pf.values()]}]
          # Timestamp type is not json serializable
          data[0]['points'][0][-1] = pf['start_date'].__str__()
  
          self.session.write_points(dataf save_portfolio(self, datetime, portfolio):
          '''
          Store in Rethinkdb a zipline.Portfolio object
          '''
          log.debug('Saving portfolio in database')
          pf = dbutils.portfolio_to_dict(portfolio)
          pf.pop('positions')
          data = [{
              "name": self.name,
              "time": datetime.strftime("%Y-%m-%d %H:%M"),
              "columns": pf.keys(),
              "points": [pf.values()]}]
          # Timestamp type is not json serializable
          data[0]['points'][0][-1] = pf['start_date'].__str__()
  
          self.session.write_points(data)
docs: Store in Rethinkdb a zipline.Portfolio object