Crash validating related model created using inline admin - django

Question

I'm creating a one-to-one model to extend the functionality of an existing model type, but I want it to only allow creating the extension model in certain cases. I enforce this constraint by throwing a ValidationError in the full_clean on the new Extended model. This works great when I create Extended models using Extended's ModelAdmin directly (it highlights the a field if it's the wrong type), but when I use StackedInline to inline Extended creation in As ModelAdmin, and A is the wrong type, the form fails to catch the ValidationError and I get the message A server error occurred. Please contact the administrator.
This is how I have the models set up:
# models.py
from django.db import models
class A(models.Model):
type = models.IntegerField(...)
class Extended(models.Model)
a = models.OneToOneField(A)
def clean_fields(self, **kwargs):
if self.a.type != 3:
raise ValidationError({'a': ["a must be of type 3"]})
super(Extended, self).clean_fields(**kwargs)
def save(self, *args, **kwargs):
self.full_clean()
super(Extended, self).save(*args, **kwargs)
# admin.py
from django.contrib import admin
class ExtendedInline(admin.StackedInline):
model = Extended
#admin.register(A)
class AAdmin(admin.ModelAdmin):
inlines = (ExtendedInline,)
The full traceback:
Traceback (most recent call last):
File "/usr/local/lib/python2.7/wsgiref/handlers.py", line 85, in run
self.result = application(self.environ, self.start_response)
File "/usr/local/lib/python2.7/site-packages/django/contrib/staticfiles/handlers.py", line 63, in __call__
return self.application(environ, start_response)
File "/usr/local/lib/python2.7/site-packages/whitenoise/base.py", line 66, in __call__
return self.application(environ, start_response)
File "/usr/local/lib/python2.7/site-packages/django/core/handlers/wsgi.py", line 189, in __call__
response = self.get_response(request)
File "/usr/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 218, in get_response
response = self.handle_uncaught_exception(request, resolver, sys.exc_info())
File "/usr/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 132, in get_response
response = wrapped_callback(request, *callback_args, **callback_kwargs)
File "/usr/local/lib/python2.7/site-packages/django/contrib/admin/options.py", line 618, in wrapper
return self.admin_site.admin_view(view)(*args, **kwargs)
File "/usr/local/lib/python2.7/site-packages/django/utils/decorators.py", line 110, in _wrapped_view
response = view_func(request, *args, **kwargs)
File "/usr/local/lib/python2.7/site-packages/django/views/decorators/cache.py", line 57, in _wrapped_view_func
response = view_func(request, *args, **kwargs)
File "/usr/local/lib/python2.7/site-packages/django/contrib/admin/sites.py", line 233, in inner
return view(request, *args, **kwargs)
File "/usr/local/lib/python2.7/site-packages/django/contrib/admin/options.py", line 1521, in change_view
return self.changeform_view(request, object_id, form_url, extra_context)
File "/usr/local/lib/python2.7/site-packages/django/utils/decorators.py", line 34, in _wrapper
return bound_func(*args, **kwargs)
File "/usr/local/lib/python2.7/site-packages/django/utils/decorators.py", line 110, in _wrapped_view
response = view_func(request, *args, **kwargs)
File "/usr/local/lib/python2.7/site-packages/django/utils/decorators.py", line 30, in bound_func
return func.__get__(self, type(self))(*args2, **kwargs2)
File "/usr/local/lib/python2.7/site-packages/django/utils/decorators.py", line 145, in inner
return func(*args, **kwargs)
File "/usr/local/lib/python2.7/site-packages/django/contrib/admin/options.py", line 1470, in changeform_view
self.save_related(request, form, formsets, not add)
File "/usr/local/lib/python2.7/site-packages/django/contrib/admin/options.py", line 1104, in save_related
self.save_formset(request, form, formset, change=change)
File "/usr/local/lib/python2.7/site-packages/django/contrib/admin/options.py", line 1092, in save_formset
formset.save()
File "/usr/local/lib/python2.7/site-packages/django/forms/models.py", line 636, in save
return self.save_existing_objects(commit) + self.save_new_objects(commit)
File "/usr/local/lib/python2.7/site-packages/django/forms/models.py", line 767, in save_new_objects
self.new_objects.append(self.save_new(form, commit=commit))
File "/usr/local/lib/python2.7/site-packages/django/forms/models.py", line 900, in save_new
obj.save()
File "/code/app/models.py", line 162, in save
self.full_clean()
File "/usr/local/lib/python2.7/site-packages/django/db/models/base.py", line 1171, in full_clean
raise ValidationError(errors)
ValidationError: {'a': [u'a must be of type 3']}
I'm currently using Django version 1.8

Answer 1

Inline foreignkeys are excluded on the calls to full_clean() that happen in modelform validators, so your ValidationError is not being caught by the form's is_valid() call.
From django/forms/models.py:
def _post_clean(self):
opts = self._meta
exclude = self._get_validation_exclusions()
try:
self.instance = construct_instance(self, self.instance, opts.fields, exclude)
except ValidationError as e:
self._update_errors(e)
# Foreign Keys being used to represent inline relationships
# are excluded from basic field value validation. This is for two
# reasons: firstly, the value may not be supplied (#12507; the
# case of providing new values to the admin); secondly the
# object being referred to may not yet fully exist (#12749).
# However, these fields *must* be included in uniqueness checks,
# so this can't be part of _get_validation_exclusions().
for name, field in self.fields.items():
if isinstance(field, InlineForeignKeyField):
exclude.append(name)
try:
self.instance.full_clean(exclude=exclude, validate_unique=False)
except ValidationError as e:
self._update_errors(e)
# Validate uniqueness if needed.
if self._validate_unique:
self.validate_unique()
They're being caught in save() instead (where you call full_clean without the exclusion), which is too late.
Move your validation to clean() instead:
def clean(self):
if self.a.type != 3:
raise ValidationError({'a': ["a must be of type 3"]})
Then there will be no need to call full_clean from your save method. This method is where any validation of this sort should go as per the docs.

Answer 2

If I change
raise ValidationError({'a': ["a must be of type 3"]})
to
raise ValidationError(["a must be of type 3"])
I don't get the crash and it appears on the inline admin.

Answer 3

I'm getting this in the a regular (non-inline) ModelAdmin, but this solution should also work.
I simply wrapped the save_model() in a try except and sent the exception message through a messages.warning()
from django.contrib import admin, messages
class ConstraintErrorBoundaryMixin:
def save_model(self, request, obj, form, change):
try:
super().save_model(request, obj, form, change)
except Exception as exc:
messages.error(request, str(exc))