#24395: Fixing #24325 (Cannot reference FK relation from inline 
ModelForm.save())
----------------------------+--------------------
     Reporter:  Starou      |      Owner:  nobody
         Type:  Bug         |     Status:  new
    Component:  Forms       |    Version:  master
     Severity:  Normal      |   Keywords:
 Triage Stage:  Unreviewed  |  Has patch:  0
Easy pickings:  0           |      UI/UX:  0
----------------------------+--------------------
 Hi devs,

 This is directly related to the closed ticket #24235 by Tim.

 The problem is that the instance of the related object (an `Author`
 instance in my example) in the formset's forms instances (`BookForm`) is
 not up-to-date.

 In the admin, the call stack is:


 {{{
 # django.contrib.admin.options.py
 ModelAdmin.changeform_view():
     ...
     new_object = self.save_form(request, form, change=not add)
     ...
     self.save_model(request, new_object, form, not add)
     self.save_related(...) [1]

     def save_related():
         self.save_formset(request, form, formset, change=change) [2]

     def save_formset(self, request, form, formset, change):
         formset.save() [3]
 }}}

 At this point [1], the main form has already been saved and the
 `new_object` (an `Author` instance) is in the database.
 This `new_object` instance is correctly bound to the formset(s)
 instance(s) but not to the formsets' forms.

 IMAO this is a bug.
 Since the `new_object` is saved in the database and all the forms have
 been validated, there is no reason to have an old instance attached to the
 ModelForm.instance attribute (which is a `Book` instance in my example).

 A basic patch could be something like that:


 {{{
 diff --git a/django/forms/models.py b/django/forms/models.py
 index 98f84a0..40217f4 100644
 --- a/django/forms/models.py
 +++ b/django/forms/models.py
 @@ -649,6 +649,7 @@ class BaseModelFormSet(BaseFormSet):
                  for form in self.saved_forms:
                      form.save_m2m()
              self.save_m2m = save_m2m
 +        self.set_forms_related_object(self.instance)
          return self.save_existing_objects(commit) +
 self.save_new_objects(commit)

      save.alters_data = True
 @@ -656,6 +657,11 @@ class BaseModelFormSet(BaseFormSet):
      def clean(self):
          self.validate_unique()

 +    def set_forms_related_object(self, obj):
 +        rel_name = self.fk.name
 +        for form in self.forms:
 +            setattr(form.instance, rel_name, obj)
 +
      def validate_unique(self):
          # Collect unique_checks and date_checks to run from all the
 forms.
          all_unique_checks = set()

 }}}

 What do you guys think ?


 This patch passed most of admin tests. But I got a segfault with the last
 master checkout either I use the patch or not.


 {{{
 stan@stanislrrasimac:tests$ PYTHONPATH=..:$PYTHONPATH python runtests.py
 admin_views -v 2
 Testing against Django installed in
 '/Users/stan/src/Django/repos/django/django/django'
 Importing application admin_views
 Creating test database for alias 'default' (':memory:')...
 ...
 test_logout_and_password_change_URLs
 (admin_views.tests.AdminViewBasicTest) ... ok
 test_multiple_sort_same_field (admin_views.tests.AdminViewBasicTest) ...
 ok
 test_named_group_field_choices_change_list
 (admin_views.tests.AdminViewBasicTest) ... ok
 test_named_group_field_choices_filter
 (admin_views.tests.AdminViewBasicTest) ... ok
 test_popup_add_POST (admin_views.tests.AdminViewBasicTest) ... ok
 test_popup_dismiss_related (admin_views.tests.AdminViewBasicTest) ... ok
 test_proxy_model_content_type_is_used_for_log_entries
 (admin_views.tests.AdminViewBasicTest) ... ok
 test_relation_spanning_filters (admin_views.tests.AdminViewBasicTest) ...
 ok
 test_resolve_admin_views (admin_views.tests.AdminViewBasicTest) ... ok
 test_sort_indicators_admin_order (admin_views.tests.AdminViewBasicTest)
 ... ok
 Segmentation fault: 11
 stan@stanislrrasimac:tests$
 }}}

--
Ticket URL: <https://code.djangoproject.com/ticket/24395>
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