Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'query_id') and self.query_id is not None:
            _dict['query_id'] = self.query_id
        if hasattr(self, 'natural_language_query'
                  ) and self.natural_language_query is not None:
            _dict['natural_language_query'] = self.natural_language_query
        if hasattr(self, 'filter') and self.filter is not None:
            _dict['filter'] = self.filter
        if hasattr(self, 'examples') and self.examples is not None:
            _dict['examples'] = [x._to_dict() for x in self.examples]
        return _dict