修改 views.py

@auth
def edit_teacher(request, nid):
    if request.method == "GET":
        obj = models.Teacher.objects.get(id=nid)
        
        # 执行后返回元组，每个元组包含传递给values_list() 调用的字段的值
        # obj_cls_list = obj.cls.all().values_list('id')
        # 把元组中的第一个字段取出，组成列表
        # id_list = list(zip(*obj_cls_list))[0]

        # 通过 values_list 函数中直接设置 flat=True 返回列表
        id_list = obj.cls.all().values_list('id', flat=True).order_by('id')
        # print(id_list)
        cls_list = models.Classes.objects.all()

        return render(request, 'edit_teacher.html', {'obj': obj, 'id_list': id_list, 'cls_list': cls_list})

    elif request.method == "POST":
        name = request.POST.get('name')
        cls_li = request.POST.getlist('cls')

        # 更新操作
        obj = models.Teacher.objects.get(id=nid)
        obj.name = name
        obj.save()

        obj.cls.set(cls_li)

        return redirect('/teacher.html')

添加 edit_teacher.html

{% extends "layout.html" %}

{% block css %}
{% endblock %}

{% block content %}
    <h1>编辑老师</h1>
    <form action="/edit_teacher-{{ obj.id }}.html" method="POST">
        <input style="display: none" type="text" id="nid" value="{{ obj.id }}" />
        <p>
            老师姓名: <input type="text" name="name" value="{{ obj.name }}" />
        </p>
        <p>
            班级:
            <select name="cls" multiple>
                <!-- 循环 cls_list -->
                {% for row in cls_list %}
                    <!--  判断 row.id 是否选中 -->
                    {% if row.id in id_list %}
                        <option value="{{ row.id }}" selected="selected">{{ row.caption }}</option>
                        {% else %}
                        <option value="{{ row.id }}">{{ row.caption }}</option>
                    {% endif %}
                {% endfor %}
            </select>
        </p>
        <input type="submit" value="提交">
    </form>
{% endblock %}


{% block js %}
    <script>
        $(function () {
            $('#menu_teacher').addClass('active');
        })
    </script>
{% endblock %}

修改 teacher.html

{% extends "layout.html" %}

{% block css %}

        .tag{
            display: inline-block;
            padding: 5px;
            border: 1px solid red;
            background-color: lightpink;
            cursor: pointer;
        }

{% endblock %}

{% block content %}

    <h1>老师列表</h1>
    <div>
        <a href="/add_teacher.html">添加</a>
    </div>
    <table border="1">
        <thead></thead>
        <tbody>
            {% for dic in teacher_list.values %}
                <tr>
                    <td>{{ dic.nid }}</td>
                    <td>{{ dic.name }}</td>
                    <td>
                        {% for c in dic.cls_list %}
                            <span class="tag" nid="{{ c.id }}">{{ c.caption }}</span>
                        {% endfor %}
                    </td>
                    <td>
                        <a href="edit_teacher-{{ dic.nid }}.html">编辑</a>
                    </td>
                </tr>
            {% endfor %}
        </tbody>
    </table>
{% endblock %}

{% block js %}
    <script>
        $(function () {
            $('#menu_teacher').addClass('active');

        })
    

    </script>
{% endblock %}

修改 urls.py

# 注意这里需要 import re_path 来试用正则表达式
from django.urls import path, re_path

urlpatterns = [

    # 添加下面几个
    path('teacher.html', views.handle_teacher),
    path('add_teacher.html', views.add_teacher),
    re_path('edit_teacher-(\d+).html', views.edit_teacher),

]