Fixed #20644 -- Add ModelFormMixin.fields to the CBV flattened index

Thanks to Tim Graham for the report and review.

django/views/generic/edit.py

@@ -89,6 +89,7 @@ class ModelFormMixin(FormMixin, SingleObjectMixin):
     """
     A mixin that provides a way to show and handle a modelform in a request.
     """
+    fields = None
 
     def get_form_class(self):
         """
@@ -109,13 +110,12 @@ class ModelFormMixin(FormMixin, SingleObjectMixin):
                 # from that
                 model = self.get_queryset().model
 
-            fields = getattr(self, 'fields', None)
-            if fields is None:
+            if self.fields is None:
                 warnings.warn("Using ModelFormMixin (base class of %s) without "
                               "the 'fields' attribute is deprecated." % self.__class__.__name__,
                               PendingDeprecationWarning)
 
-            return model_forms.modelform_factory(model, fields=fields)
+            return model_forms.modelform_factory(model, fields=self.fields)
 
     def get_form_kwargs(self):
         """

docs/ref/class-based-views/flattened-index.txt

@@ -177,6 +177,7 @@ CreateView
 
 * :attr:`~django.views.generic.base.TemplateResponseMixin.content_type`
 * :attr:`~django.views.generic.detail.SingleObjectMixin.context_object_name` [:meth:`~django.views.generic.detail.SingleObjectMixin.get_context_object_name`]
+* :attr:`~django.views.generic.edit.ModelFormMixin.fields`
 * :attr:`~django.views.generic.edit.FormMixin.form_class` [:meth:`~django.views.generic.edit.FormMixin.get_form_class`]
 * :attr:`~django.views.generic.base.View.http_method_names`
 * :attr:`~django.views.generic.edit.FormMixin.initial` [:meth:`~django.views.generic.edit.FormMixin.get_initial`]
@@ -216,6 +217,7 @@ UpdateView
 
 * :attr:`~django.views.generic.base.TemplateResponseMixin.content_type`
 * :attr:`~django.views.generic.detail.SingleObjectMixin.context_object_name` [:meth:`~django.views.generic.detail.SingleObjectMixin.get_context_object_name`]
+* :attr:`~django.views.generic.edit.ModelFormMixin.fields`
 * :attr:`~django.views.generic.edit.FormMixin.form_class` [:meth:`~django.views.generic.edit.FormMixin.get_form_class`]
 * :attr:`~django.views.generic.base.View.http_method_names`
 * :attr:`~django.views.generic.edit.FormMixin.initial` [:meth:`~django.views.generic.edit.FormMixin.get_initial`]