Fixed #25097 -- Added BaseModelFormSet.delete_existing() hook.

        """Saves and returns an existing model instance for the given form."""

        return form.save(commit=commit)

    def delete_existing(self, obj, commit=True):

        """Deletes an existing model instance."""

        if commit:

            obj.delete()

    def save(self, commit=True):

        """Saves model instances for every form, adding and changing instances

        as necessary, and returns the list of instances.

                if obj.pk is None:

                    continue

                self.deleted_objects.append(obj)

                if commit:

                    obj.delete()

                self.delete_existing(obj, commit=commit)

            elif form.has_changed():

                self.changed_objects.append((obj, form.changed_data))

                saved_instances.append(self.save_existing(form, obj, commit=commit))