Return a json dictionary representing this model.

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