如何将css类添加到由RelatedFieldWidgetWrapper生成的django表单字段

from django import forms
from django.contrib.admin.widgets import RelatedFieldWidgetWrapper
from django.contrib.auth.models import User

class CustomForm(forms.ModelForm):
    class Meta:
        model = User
        fields = '__all__'

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        # 获取外键字段对应的表单字段
        field = self.fields['field_name']

        # 创建RelatedFieldWidgetWrapper实例，并添加自定义的CSS类
        wrapper = RelatedFieldWidgetWrapper(
            field.widget, 
            field.queryset.model._meta.get_field(field.queryset.field.name).remote_field,
            admin_site
        )
        wrapper.widget.attrs.update({'class': 'custom-class'})

        # 将自定义的widget应用到字段
        field.widget = wrapper

# 在视图中使用自定义表单类
def my_view(request):
    form = CustomForm()
    return render(request, 'my_template.html', {'form': form})