Return a json dictionary representing this model.

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