Return a json dictionary representing this model.

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