用第二种方式需要加上下面的这个：
三、判断用户民是不存在，存在就不添加了
from django.core.exceptions import ValidationError
initial 修改时用到
五、自定义配置
1、在settings中：
2、导入settings
from django.conf import settings
3.使用
多对多修改两种方式（用Form）
def editteacher(request,nid):
obj = models.UserInfo.objects.filter(id=nid, ut_id=1).first()
# print(obj.username)
if not obj:
return redirect("/teacherindex/")
if request.method=="GET":
print([obj.id for obj in obj.teacher_classes.all()]) #[2] 拿到select框的id是为了要做默认显示的
form = TeacherForm(initial={"username":obj.username,"password":obj.password,"email":obj.email,"teacher_classes":[obj.id for obj in obj.teacher_classes.all()]}) #就让显示一个input框，并且带有原来哪一行的内容
return render(request, "editteacher.html", {"form":form})
else:
form = TeacherForm(data=request.POST)
if form.is_valid():#开始校验,注意这要加括号
cls_list = form.cleaned_data.pop("teacher_classes")
print(cls_list)
models.UserInfo.objects.filter(id=nid).update(**form.cleaned_data)
obj.teacher_classes.set(cls_list) #更新第三张表
return redirect("/teacherindex/")
else:
return render(request, "editteacher.html", {"form":form})
多对多添加（用Form）
def addteacher(request):
if request.method=="GET":
form = TeacherForm() #只是让显示一个input框
return render(request, "addteacher.html", {"form":form})
else:
form = TeacherForm(data=request.POST)
# print(form) #<QuerySet [<UserInfo: UserInfo object>, <UserInfo: UserInfo object>, <UserInfo: UserInfo object>]>
if form.is_valid():# 开始验证
# print(‘执行成功‘,form.cleaned_data) # 所有匹配成功，字典
# # {‘username‘: ‘ghf‘, ‘password‘: ‘435‘, ‘email‘: ‘fddfg@qq.com‘, ‘teacher_classes‘: [‘4‘]}
cls_list = form.cleaned_data.pop("teacher_classes")
print("============id",cls_list)
form.cleaned_data[‘ut_id‘] = 1
#创建新老师的对象
teacher_obj = models.UserInfo.objects.create(**form.cleaned_data)
#创建新老师和班级的关系
teacher_obj.teacher_classes.add(*cls_list) #以前添加的是对象，现在也可以吧id添加进去
return redirect("/teacherindex/")
else:
# print("=====?",form.errors,type(form.errors))#返回失败的结果
# print(form.errors["username"][0]) #拿到返回失败的结果，渲染到页面
return render(request, "addteacher.html", {"form":form})
原文：https://www.cnblogs.com/yanxiaoge/p/10588804.html