Django 错误不为Null参与者未通过测验\u测验。主题\u id

我有这张桌子

class Quiz(models.Model):
    quiz_title = models.CharField(max_length=300)
    num_questions = models.IntegerField(default=0)
    subject=models.ForeignKey(Subject,on_delete=models.CASCADE,blank=True,null=True)

    def __str__(self):
        return self.quiz_title

在这个表中，你们可以看到我有主题外键，比如当我创建测验时，选择我要创建测验的主题外键

class CreateQuizForm(forms.Form):
    quiz_name = forms.CharField(max_length=50, label="Quiz Name", widget=forms.TextInput(attrs={'class': 'quiz_name_box'}))
    num_questions = forms.IntegerField(label="Number of Questions", widget=forms.NumberInput(attrs={'class': 'num_questions_box'}))
    speed = forms.ModelChoiceField(queryset=Subject.objects.all())
    def clean_name(self):
        data = self.cleaned_data['quiz_name']

        # Check if quiz name is unique.
        if data == "Default_Name":
            raise ValidationError(_('Invalid name - Must be Unique'))

        # returns cleaned data
        return data

这是我的CreateQuizForm 当我选择主题时，它显示出完整性，无贡献错误