Return a json dictionary representing this model.

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