Django ModelForm更新函数返回错误

# models.py
from django.db import models

class MyModel(models.Model):
    name = models.CharField(max_length=100)
    description = models.TextField()

# forms.py
from django import forms
from .models import MyModel

class MyModelForm(forms.ModelForm):
    class Meta:
        model = MyModel
        fields = ['name', 'description']

# views.py
from django.shortcuts import render, get_object_or_404, redirect
from .models import MyModel
from .forms import MyModelForm

def update_model(request, pk):
    instance = get_object_or_404(MyModel, pk=pk)
    if request.method == 'POST':
        form = MyModelForm(request.POST, instance=instance)
        if form.is_valid():
            form.save()
            return redirect('success_url')  # 重定向到成功页面
        else:
            # 处理表单验证错误
            context = {'form': form}
            return render(request, 'update_form.html', context)
    else:
        form = MyModelForm(instance=instance)
        context = {'form': form}
        return render(request, 'update_form.html', context)