Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'id') and self.id is not None:
            _dict['id'] = self.id
        if hasattr(self, 'metadata') and self.metadata is not None:
            _dict['metadata'] = self.metadata
        if hasattr(self, 'collection_id') and self.collection_id is not None:
            _dict['collection_id'] = self.collection_id
        if hasattr(self,
                   'result_metadata') and self.result_metadata is not None:
            _dict['result_metadata'] = self.result_metadata._to_dict()
        if hasattr(self, 'title') and self.title is not None:
            _dict['title'] = self.title
        if hasattr(self, '_additionalProperties'):
            for _key in self._additionalProperties:
                _value = getattr(self, _key, None)
                if _value is not None:
                    _dict[_key] = _value
        return _dict