Return a json dictionary representing this model.

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