docs: def mongo_query(self):
          """
          Convert a DateRange into a MongoDb query string. FIXME: Mongo can only handle
          datetimes in queries, so we should make this handle the case where start/end are
          datetime.date and extend accordingly (being careful about the interval logic).
          """
          comps = {OPEN_CLOSED: ('t', 'te'), OPEN_OPEN: ('t', 't'),
                   CLOSED_OPEN: ('te', 't'), CLOSED_CLOSED: ('te', 'te')}
          query = {}
          comp = comps[self.interval]
          if self.start:
              query['$g' + comp[0]] = self.start
          if self.end:
              query['$l' + comp[1]] = self.end
          return query