Return a json dictionary representing this model.

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