Return a json dictionary representing this model.

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