Django将多个选项添加到现有多个

我试图将表单中的许多选项添加到现有的许多选项中 我在views.py中尝试过

        instance = form.save(commit=False)
        id.attendees.add(instance.attendees)
        instance.save()

        instance = form.save(commit=False)
        for choice in instance.attendees:
            id.attendees.add(choice)
        form.save()

和在views.py中

        instance = form.save(commit=False)
        id.attendees.add(instance.attendees)
        instance.save()

        instance = form.save(commit=False)
        for choice in instance.attendees:
            id.attendees.add(choice)
        form.save()

整个代码是

forms.py

class LessonAddForm(forms.ModelForm):
def __init__(self, *args, **kwargs):
    user = kwargs.pop('user', None)
    super(LessonAddForm, self).__init__(*args, **kwargs)
    self.fields['attendees'].queryset = CustomUser.objects.filter(
        Q(username=user) |
        Q(customuser__username=user)
    )

class Meta:
    model = models.Lessons
    fields = ['attendees']

views.py

def lessonRecord(request,id):
    if request.method == "POST":
        id = Lessons.objects.get(id=id)
        form = forms.LessonAddForm(request.POST,user=request.user,instance=id)
        if form.is_valid():
            instance = form.save(commit=False)
            for choice in instance.attendees:
                id.attendees.add(choice)
            form.save()
            return redirect('lessons:lessonsView')
    else:
        id = Lessons.objects.get(id=id)
        #form = forms.LessonAddForm(instance=id)
        form = forms.LessonAddForm(instance=id , user=request.user)
    return render(request,'lessons/lesson_record.html',{'id':id,'form':form})

我只是不知道如何将表单中的选择添加到现有的选择中