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