Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'interval') and self.interval is not None:
            _dict['interval'] = self.interval
        if hasattr(self, 'event_type') and self.event_type is not None:
            _dict['event_type'] = self.event_type
        if hasattr(self, 'results') and self.results is not None:
            _dict['results'] = [x._to_dict() for x in self.results]
        return _dict