Issue with save/save_model/clean from admin (django 1.6.3dev)

[Stefano Crosta]

Hello!

I'm trying to understand if this is a bug, or a not clearly documented feature.

I have a simple model where one field should be calculated at save. eg.

class MyModel(models.Model):

    a = models.CharField(max_length=255) 

    b = models.CharField(max_length=255, blank = True)

Now, if I create an instance in the admin and I try to save leaving `b` empty (that's ok because it's blank=True) I get a ValueError:

Cannot assign None: "app.MyModel" does not allow null values.

Now, I would expect to be able to set `b` to my calculated value in one of the following places, according to the documentation:

- overriding save() in the model class

- in the clean() method in the model class

- with a custom save_model() in the admin class

Instead, none of those locations are reached, and I get the ValueError from 

django/db/models/fields/related.py in __set__

326.             return rel_obj

328.     def __set__(self, instance, value):

329.         # If null=True, we can assign null here, but otherwise the value needs

330.         # to be an instance of the related class.

331.         if value is None and self.field.null == False:

332.             raise ValueError('Cannot assign None: "%s.%s" does not allow null values.' %

333.                                 (instance._meta.object_name, self.field.name))

now, also checking the stack (and even debugging) it seems like this comes earlier than save.

My question: Is this somehow expected, is this a bug/a regression?

If it's expected, I'll find another workaround; if it's a bug, I can add a ticket.

Thanks!

Stefano

Stacktrack

• /home/3h/django/django/contrib/admin/options.py in add_view

  1113.             if form.is_valid():

        ...
  ▶ Local vars
• /home/3h/django/django/forms/forms.py in is_valid

  129.         return self.is_bound and not bool(self.errors)

       ...
  ▶ Local vars
• /home/3h/django/django/forms/forms.py in errors

  121.             self.full_clean()

       ...
  ▶ Local vars
• /home/3h/django/django/forms/forms.py in full_clean

  275.         self._post_clean()

       ...
  ▶ Local vars
• /home/3h/django/django/forms/models.py in _post_clean

  396.         self.instance = construct_instance(self, self.instance, opts.fields, opts.exclude)

       ...
  ▶ Local vars
• /home/3h/django/django/forms/models.py in construct_instance

  57.             f.save_form_data(instance, cleaned_data[f.name])

      ...
  ▶ Local vars
• /home/3h/django/django/db/models/fields/__init__.py in save_form_data

  516.         setattr(instance, self.name, data)

       ...
  ▶ Local vars
• /home/3h/django/django/db/models/fields/related.py in __set__

  333.                                 (instance._meta.object_name, self.field.name))

       ...
  ▶ Local vars

[Jonathan Pentecost]

I _think_ your problem might lie in the confusion between "blank" and "null"; taken from https://docs.djangoproject.com/en/dev/ref/models/fields/#blank; 

 null is purely database-related, whereas blank is validation-related. If a field has blank=True, form validation will allow entry of an empty value. If a field has blank=False, the field will be required.

[Shai Berger]

Hi Stefano,

You are leaving out important details -- the error you are getting is clearly
about a "related field" (ForeignKey, OneToOne or ManyToMany), but there is no
such field in the model you provided.

I suspect it is more likely to be a usage issue rather than a problem in
Django code or documentation, and therefore I suggest you bring it up (with
the missing details) at the django-users list.

HTH,
Shai.

On Tuesday 25 February 2014 05:17:21 Stefano Crosta wrote:
> Hello!
>
> I'm trying to understand if this is a bug, or a not clearly documented
> feature.
>
> I have a simple model where one field should be calculated at save. eg.
>
> class MyModel(models.Model):
> a = models.CharField(max_length=255)
> b = models.CharField(max_length=255, blank = True)
>
> Now, if I create an instance in the admin and I try to save leaving `b`
> empty (that's ok because it's blank=True) I get a ValueError:
>
> Cannot assign None: "app.MyModel" does not allow null values.
>
> Now, I would expect to be able to set `b` to my calculated value in one of
> the following places, according to the documentation:
>
>
> - overriding save() in the model class
> - in the clean() method in the model class
> - with a custom save_model() in the admin class
>
> Instead, none of those locations are reached, and I get the ValueError from
>
> django/db/models/fields/related.py in __set__
>

> 1.
>
> return rel_obj
>
> 2.
>
> 3.
>
> def __set__(self, instance, value):
>
> 4.

>
> # If null=True, we can assign null here, but otherwise the value
> needs
>

> 5.

>
> # to be an instance of the related class.
>

> 6.

>
> if value is None and self.field.null == False:
>

> 7.

>
> raise ValueError('Cannot assign None: "%s.%s" does not allow
> null values.' %
>
>
>

> 1.

>
> (instance._meta.object_name,
> self.field.name))
>
>
>
> now, also checking the stack (and even debugging) it seems like this comes
> earlier than save.
>
>

> *My question: **Is this somehow expected, is this a bug/a regression?*

>
> If it's expected, I'll find another workaround; if it's a bug, I can add a
> ticket.
>
>
> Thanks!
>
> Stefano
>
>
>
> Stacktrack
>
>

> - /home/3h/django/django/contrib/admin/options.py in add_view
> 1.
>
> if form.is_valid():
>
> ...
> ▶ Local vars <http://grigio.vm:8000/admin/ipr/ipr/add/#>
> - /home/3h/django/django/forms/forms.py in is_valid
> 1.

>
> return self.is_bound and not bool(self.errors)
>
> ...

> ▶ Local vars <http://grigio.vm:8000/admin/ipr/ipr/add/#>
> - /home/3h/django/django/forms/forms.py in errors
> 1.
>
> self.full_clean()
>
> ...
> ▶ Local vars <http://grigio.vm:8000/admin/ipr/ipr/add/#>
> - /home/3h/django/django/forms/forms.py in full_clean
> 1.
>
> self._post_clean()
>
> ...
> ▶ Local vars <http://grigio.vm:8000/admin/ipr/ipr/add/#>
> - /home/3h/django/django/forms/models.py in _post_clean
> 1.

>
> self.instance = construct_instance(self, self.instance,
> opts.fields, opts.exclude)
>
> ...

> ▶ Local vars <http://grigio.vm:8000/admin/ipr/ipr/add/#>
> - /home/3h/django/django/forms/models.py in construct_instance
> 1.

>
> f.save_form_data(instance, cleaned_data[f.name])
>
> ...

> ▶ Local vars <http://grigio.vm:8000/admin/ipr/ipr/add/#>
> - /home/3h/django/django/db/models/fields/__init__.py in save_form_data
> 1.
>
> setattr(instance, self.name, data)
>
> ...
> ▶ Local vars <http://grigio.vm:8000/admin/ipr/ipr/add/#>
> - /home/3h/django/django/db/models/fields/related.py in __set__
> 1.
>
> (instance._meta.object_name,
> self.field.name))
>
> ...
> ▶ Local vars <http://grigio.vm:8000/admin/ipr/ipr/add/#>