Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'word_count') and self.word_count is not None:
            _dict['word_count'] = self.word_count
        if hasattr(self,
                   'character_count') and self.character_count is not None:
            _dict['character_count'] = self.character_count
        if hasattr(self, 'translations') and self.translations is not None:
            _dict['translations'] = [x._to_dict() for x in self.translations]
        return _dict