A ModelForm with a blank=True, null=False ForeignKey doesn't validate
|Reported by:||James Aylett||Owned by:||nobody|
|Has patch:||no||Needs documentation:||no|
|Needs tests:||no||Patch needs improvement:||no|
For the following:
class Collection(models.Model): created_by = models.ForeignKey(User, null=False, blank=True) # and some other fields
(with a pre_save hook that populates
created_by if it isn't already set), if on creation in the admin the HTML input for
created_by is left empty (raw id) or unselected (non-raw id ie dropdown of all users), Django will attempt to set the field to
None, which will result in a field validation failure:
File "/home/james/projects/artfinder/ENV/lib/python2.6/site-packages/django/core/handlers/base.py" in get_response 111. response = callback(request, *callback_args, **callback_kwargs) File "/home/james/projects/artfinder/ENV/lib/python2.6/site-packages/django/contrib/admin/options.py" in wrapper 366. return self.admin_site.admin_view(view)(*args, **kwargs) File "/home/james/projects/artfinder/ENV/lib/python2.6/site-packages/django/utils/decorators.py" in _wrapped_view 91. response = view_func(request, *args, **kwargs) File "/home/james/projects/artfinder/ENV/lib/python2.6/site-packages/django/views/decorators/cache.py" in _wrapped_view_func 89. response = view_func(request, *args, **kwargs) File "/home/james/projects/artfinder/ENV/lib/python2.6/site-packages/django/contrib/admin/sites.py" in inner 196. return view(request, *args, **kwargs) File "/home/james/projects/artfinder/ENV/lib/python2.6/site-packages/django/utils/decorators.py" in _wrapper 25. return bound_func(*args, **kwargs) File "/home/james/projects/artfinder/ENV/lib/python2.6/site-packages/django/utils/decorators.py" in _wrapped_view 91. response = view_func(request, *args, **kwargs) File "/home/james/projects/artfinder/ENV/lib/python2.6/site-packages/django/utils/decorators.py" in bound_func 21. return func(self, *args2, **kwargs2) File "/home/james/projects/artfinder/ENV/lib/python2.6/site-packages/django/db/transaction.py" in inner 209. return func(*args, **kwargs) File "/home/james/projects/artfinder/ENV/lib/python2.6/site-packages/django/contrib/admin/options.py" in add_view 937. if form.is_valid(): File "/home/james/projects/artfinder/ENV/lib/python2.6/site-packages/django/forms/forms.py" in is_valid 124. return self.is_bound and not bool(self.errors) File "/home/james/projects/artfinder/ENV/lib/python2.6/site-packages/django/forms/forms.py" in _get_errors 115. self.full_clean() File "/home/james/projects/artfinder/ENV/lib/python2.6/site-packages/django/forms/forms.py" in full_clean 272. self._post_clean() File "/home/james/projects/artfinder/ENV/lib/python2.6/site-packages/django/forms/models.py" in _post_clean 309. self.instance = construct_instance(self, self.instance, opts.fields, opts.exclude) File "/home/james/projects/artfinder/ENV/lib/python2.6/site-packages/django/forms/models.py" in construct_instance 51. f.save_form_data(instance, cleaned_data[f.name]) File "/home/james/projects/artfinder/ENV/lib/python2.6/site-packages/django/db/models/fields/__init__.py" in save_form_data 454. setattr(instance, self.name, data) File "/home/james/projects/artfinder/ENV/lib/python2.6/site-packages/django/db/models/fields/related.py" in __set__ 362. (instance._meta.object_name, self.field.name)) Exception Type: ValueError at /admin/galleries/collection/add/ Exception Value: Cannot assign None: "Collection.created_by" does not allow null values.
I would expect this not to happen, allowing the presave hook to step in and do the right thing. If there's an obvious solution that I just haven't noticed, or no solution, then the rest of this can be skipped.
I believe that the following note in
BaseModelForm._get_validation_exclusions() explains what's going on:
Note: don't exclude the field from
validation if the model field allows blanks. If it does, the blank
value may be included in a unique check, so cannot be excluded
However I cannot find anything in the documentation that would suggest this behaviour. The current suggested solution, I think, would be to have the field explicitly listed in the
ModelForm subclass and in
excludes in that form. That would be something like:
class CollectionAdminForm(forms.ModelForm): # need to exclude created_by, but explicitly list it as a field created_by = # SOMETHING class Meta: model = Collection exclude = ('created_by',)
however there's nothing in the documentation to suggest what the SOMETHING should be (ie what form field type should be used for
created_by). If I try to use a
ModelChoiceField, it doesn't populate it from the instance we're editing. Worse, because I'm linking to
User I really want this to be a raw id field, and that seems to be utterly magical and doesn't have an actual form field type to represent it.
At this point I think I've convinced myself that Django cannot do what I want, ie there's no simple way of configuring the admin to allow a presave-populated
ForeignKey to be settable on creation but to fall back to the presave population.
Change History (3)
comment:2 Changed 4 years ago by
|Component:||Documentation → Forms|
|Summary:||blank=True, null=False ForeignKey fields populated by presave hooks cannot be edited cleanly in the admin → A ModelForm with a blank=True, null=False ForeignKey doesn't validate|
|Triage Stage:||Unreviewed → Accepted|
|Type:||Uncategorized → Bug|