Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'total') and self.total is not None:
            _dict['total'] = self.total
        if hasattr(self, 'pending') and self.pending is not None:
            _dict['pending'] = self.pending
        if hasattr(self, 'successful') and self.successful is not None:
            _dict['successful'] = self.successful
        if hasattr(self, 'failed') and self.failed is not None:
            _dict['failed'] = self.failed
        return _dict