自定义ModelForm__init__后无法使用instance参数的问题

尝试为已有Course条目创建编辑表单，需限制ModelForm中teacher字段的可选用户范围，但使用instance=Course实例初始化表单加载预填数据时，出现错误：

TypeError at /courses/course/asdd
CourseForm.__init__() got an unexpected keyword argument 'instance'

相关代码 #

view.py #

@csrf_exempt
@login_required
def addCourse(request, course_id = None):
    #initialize data
    user = request.user
    school = user.school

    #check if submition is POST
    if request.method == 'POST':
        #if so check if user is linked to a school
        if user.school:
            #check if user has admin access to add courses
            if user.is_admin and user in school.admins:
                #get all form information
                # check if form data is valid
                if form.is_valid():
                    # save the form data to model
                    form.save()       
    
        return HttpResponseRedirect(reverse("courses", 'school'))
            
    #if GET check if user has a related school
    elif user.school:
        #check if user has admin access
        if user.is_admin:
            #check for course code, if so auto fill course info
            if course_id != None:
                #generate filled form
                    course = Course.objects.get(code=course_id)
                    form = CourseForm(instance=course, school=school)
                    return render(request, "app/courses/addCourse.html", {'form': form})
            #if no course code, generate blank form
            else:
                #generate form with school object to limit teacher options to school related users
                form = CourseForm(school)
                return render(request, "app/courses/addCourse.html", {'form': form})
                
    #if all checks fail. Redirect.
    return HttpResponseRedirect(reverse("index"))

表单代码 #

class CourseForm(forms.ModelForm):  

    class Meta():
        model = Course
        fields = ('teacher','name','subject','level', 'description', 'start_time', 'end_time', 'start_date', 'end_date')

        widgets = {
            'start_date': forms.DateInput(attrs={'type': 'date'}),
            'end_date': forms.DateInput(attrs={'type': 'date'}),
            'start_time': forms.TimeInput(attrs={'type': 'time'}),
            'end_time': forms.TimeInput(attrs={'type': 'time'}),
        }
    def __init__(self, school, course=None):
        super(CourseForm, self).__init__()
        self.fields['teacher'] = forms.ModelChoiceField(widget=forms.Select, queryset=User.objects.filter(school=school, is_teacher=True))

补充：移除自定义__init__方法后，instance参数可正常使用，但无法将teacher字段的查询集限制为关联学校的用户。

1. 修改表单的__init__方法，兼容ModelForm默认参数 #

自定义__init__需要接收所有ModelForm支持的参数（包括instance），并传递给父类，同时只需修改teacher字段的查询集即可，无需重新定义字段：

class CourseForm(forms.ModelForm):  

    class Meta():
        model = Course
        fields = ('teacher','name','subject','level', 'description', 'start_time', 'end_time', 'start_date', 'end_date')

        widgets = {
            'start_date': forms.DateInput(attrs={'type': 'date'}),
            'end_date': forms.DateInput(attrs={'type': 'date'}),
            'start_time': forms.TimeInput(attrs={'type': 'time'}),
            'end_time': forms.TimeInput(attrs={'type': 'time'}),
        }
    
    def __init__(self, school, *args, **kwargs):
        # 先调用父类初始化逻辑，处理instance等参数
        super().__init__(*args, **kwargs)
        # 限制teacher字段的可选用户范围
        self.fields['teacher'].queryset = User.objects.filter(school=school, is_teacher=True)

2. 调整视图中的表单调用方式 #

• 编辑场景（有course_id）：
  将参数调整为关键字参数或符合定义的顺序：

  form = CourseForm(school=school, instance=course)
  

  或者：

  form = CourseForm(school, instance=course)
  

• 新增场景：
  原代码form = CourseForm(school)可保留，或改为form = CourseForm(school=school)增强可读性。

3. 修复POST请求中的表单初始化bug #

当前POST分支未初始化form对象就判断form.is_valid()，会导致报错，需补上：

if request.method == 'POST':
    if user.school and user.is_admin and user in school.admins:
        # 初始化表单，传入school和POST数据
        form = CourseForm(school, request.POST)
        if form.is_valid():
            form.save()
    return HttpResponseRedirect(reverse("courses", args=['school']))

内容的提问来源于stack exchange，提问作者jimkwy