Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'entity') and self.entity is not None:
            _dict['entity'] = self.entity
        if hasattr(self, 'location') and self.location is not None:
            _dict['location'] = self.location
        if hasattr(self, 'value') and self.value is not None:
            _dict['value'] = self.value
        if hasattr(self, 'confidence') and self.confidence is not None:
            _dict['confidence'] = self.confidence
        if hasattr(self, 'metadata') and self.metadata is not None:
            _dict['metadata'] = self.metadata
        if hasattr(self, 'groups') and self.groups is not None:
            _dict['groups'] = [x._to_dict() for x in self.groups]
        if hasattr(self, '_additionalProperties'):
            for _key in self._additionalProperties:
                _value = getattr(self, _key, None)
                if _value is not None:
                    _dict[_key] = _value
        return _dict