Return a json dictionary representing this model.

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