Django models ValueError:无法分配"&书信电报；类名：V>&引用；：&引用；Teachername.classname“；必须是一个"；章节名称“；例子

My models.py：

class Classname(models.Model):  

    class_name = models.CharField(max_length=8) 

    def __str__(self):  
        return self.class_name  

class Sectionname(models.Model):  

    class_name = models.ForeignKey(Classname)  
    section_name = models.CharField(max_length=1, default='A')  
    def __str__(self):  
        return self.section_name  

class Teachername(models.Model):  
    classname = models.ForeignKey(Sectionname)
    secname = models.ForeignKey(Classname)  
    teachname = models.CharField(max_length=50)  
    def __str__(self):  
        return self.teachname  

my views.py：

def add_teach(request):  
    if request.method == 'POST':  
        form = TeachernameForm(request.POST)  
         if form.is_valid():  
            form.save()  
            return HttpResponseRedirect(reverse('student:listteach'))  
        else:  
            print(form.errors)  
    else:  
        form = TeachernameForm()  
    return render(request, 'add_teach.html', {'form': form},)  

my form.py：

class TeachernameForm(ModelForm):  
    classname = forms.ModelChoiceField(queryset=Classname.objects.all())  
    secname = forms.ModelChoiceField(queryset=Sectionname.objects.all())  
    class Meta:  
        model = Teachername  
        fields = ('classname', 'secname', 'teachname',)  

当我提交教师表格时。我得到了上面的错误。我尝试了很多技巧，但没有找到解决办法。请找个人给我建议一下

class Teachername(models.Model):  
    classname = models.ForeignKey(Sectionname)
    secname = models.ForeignKey(Classname) 

这可能是：

    classname = models.ForeignKey(Classname)
    secname = models.ForeignKey(Sectionname)