如何使用相同的django表单编辑/添加对象?

嘿,我已经到处找过了(特别是这个问题: Django edit form based on add form? )但我就是做不到。

我遇到的问题是窗体总是创建一个新对象,而不是修改现有对象。

这是我的密码:

def new_task(request, task_id=None):

if task_id is not None:
    task  = Task.objects.get(pk=task_id)
else:
    task = Task()

if request.method == 'POST': # If the form has been submitted...
    form = TaskForm(request.POST, instance=task)
    if form.is_valid():
        form.save();
        return tasks(request, 'Task #%s created successfully.' % (task.id))
else:
    form  = TaskForm(instance=task)

return custom_render('user/new_task.html',
                     {'form': form},
                     request);

关于我遗漏的东西有什么线索吗?谢谢

class TaskForm(ModelForm):

description = CharField(max_length = 1500,
    widget= forms.Textarea(attrs={'class':'task-description'}),
    required=True)

class Meta:
    model = Task