Return a json dictionary representing this model.

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