无法在Django中使用自定义表单更新自定义模型

from django import forms
from .models import CustomModel

class CustomForm(forms.ModelForm):
    # 定义需要更新的模型字段
    field1 = forms.CharField()
    field2 = forms.IntegerField()

    class Meta:
        model = CustomModel
        fields = ['field1', 'field2']

from django.shortcuts import render
from .forms import CustomForm

def update_model(request, model_id):
    # 获取需要更新的模型实例
    model_instance = CustomModel.objects.get(id=model_id)

    if request.method == 'POST':
        # 创建自定义表单实例，并传入POST数据和模型实例
        form = CustomForm(request.POST, instance=model_instance)
        
        if form.is_valid():
            # 保存更新后的数据
            form.save()
            return render(request, 'success.html')
    else:
        # 创建自定义表单实例，并传入模型实例
        form = CustomForm(instance=model_instance)

    return render(request, 'update.html', {'form': form})