在我的測驗應用程序中，每個用戶都可以進行多次嘗試。我的模型設置如下：class Quiz(models.Model):    title = models.CharField(max_length=15)    slug = models.SlugField(blank=True)    questions_count = models.IntegerField(default=0)class Question(models.Model):    quiz = models.ForeignKey(Quiz, on_delete=models.CASCADE)    label = models.CharField(max_length=1000)class Choice(models.Model):    question = models.ForeignKey(Question, on_delete=models.CASCADE)    answer = models.CharField(max_length=100)    is_correct = models.BooleanField('Correct answer', default=False)class QuizTaker(models.Model):    user = models.ForeignKey(User, on_delete=models.CASCADE)    quiz = models.ForeignKey(Quiz, on_delete=models.CASCADE)    correct_answers = models.IntegerField(default=0)    completed = models.BooleanField(default=False)    attempt_number = models.PositiveIntegerField(default=0)當我嘗試確定給定的測驗是否已完成時，我的序列化程序中出現錯誤：class MyQuizListSerializer(serializers.ModelSerializer):    questions_count = serializers.SerializerMethodField()    completed = serializers.SerializerMethodField()    progress = serializers.SerializerMethodField()    score = serializers.SerializerMethodField()    class Meta:        model = Quiz        fields = ['id', 'title', 'type_of_content', 'song', 'slug',         'questions_count', 'completed', 'score', 'progress']        read_only_fields = ['questions_count', 'completed', 'progress']    def get_completed(self, obj):        try:            quiztaker = QuizTaker.objects.filter(user=self.context['request'].user, quiz=obj)            for attempt in quiztaker:                return quiztaker.completed   #the error comes from this line        except QuizTaker.DoesNotExist:            return None誰能告訴我為什么我會收到此錯誤？我正在過濾，因為用戶可以進行多次嘗試，因此我得到一個查詢集，因此我必須循環遍歷它。該QuizTaker模型確實有一個completed字段，那么問題是什么？