Django throws ValueError

Question

When I type form.is_valid() (being form a modelForm of a model I made) django throws me the following exception:

ValueError: Cannot assign None: "Membership.member" does not allow null values.

But when I validate again, I get that the form is true.

>>> User.objects.get(id=2L)
<User: b.tollez@nuberson.travel>
>>> data={'member': 2L, 'membership_role': 3L, 'read_access': True, 'research': 2L}
>>> f=ResearchFormStepTwo(data=data)
>>> f.is_valid()
Traceback (most recent call last):
  File "<input>", line 1, in <module>
  File "/home/django/virtualenvs/patman/lib/python2.7/site-packages/django/forms/forms.py", line 126, in is_valid
    return self.is_bound and not bool(self.errors)
  File "/home/django/virtualenvs/patman/lib/python2.7/site-packages/django/forms/forms.py", line 117, in _get_errors
    self.full_clean()
  File "/home/django/virtualenvs/patman/lib/python2.7/site-packages/django/forms/forms.py", line 274, in full_clean
    self._post_clean()
  File "/home/django/virtualenvs/patman/lib/python2.7/site-packages/django/forms/models.py", line 315, in _post_clean
    self.instance = construct_instance(self, self.instance, opts.fields, opts.exclude)
  File "/home/django/virtualenvs/patman/lib/python2.7/site-packages/django/forms/models.py", line 52, in construct_instance
    f.save_form_data(instance, cleaned_data[f.name])
  File "/home/django/virtualenvs/patman/lib/python2.7/site-packages/django/db/models/fields/__init__.py", line 466, in save_form_data
    setattr(instance, self.name, data)
  File "/home/django/virtualenvs/patman/lib/python2.7/site-packages/django/db/models/fields/related.py", line 401, in __set__
    (instance._meta.object_name, self.field.name))
ValueError: Cannot assign None: "Membership.member" does not allow null values.
>>> f.is_valid()
True

The stack trace does not include code of my own (only django libraries).

My model is:

class Membership(models.Model):
    member=models.ForeignKey(PatmanUser,on_delete=models.CASCADE,db_index=False)
    research=models.ForeignKey(Research,on_delete=models.CASCADE)
    membership_role=models.ForeignKey(Role,default=Role.get_sentinel_role,on_delete=models.SET_DEFAULT,db_index=False)
    read_access=models.BooleanField(default=True,db_index=False)
    write_access=models.BooleanField(default=False,db_index=False)

    def clean(self, *args, **kwargs):
            if self.pk is None and self.read_access==False and self.write_access==False:
                    raise ValidationError('No es posible pertenecer a un estudio sin permisos definidos')

            if Membership.objects.filter(member=self.member,research=self.research).count()>1:
                    raise ValidationError('No puede haber dos pertenencias de un usuario a un estudio')

            if not self.member.is_active:
                    raise ValidationError('El usuario no esta activo')

            if self.read_access==False and self.write_access==True:
                    raise ValidationError('No puede tener permisos de escritura y no de lectura')

            super(Membership,self).clean(*args, **kwargs)

    def save(self, *args, **kwargs):
            if self.pk is not None and self.read_access==False and self.write_access==False:
                    real_object=Membership.objects.get(pk=self.pk)
                    real_object.delete()
            else:
                    super(Membership, self).save(*args, **kwargs)

    class Meta:
            verbose_name = "Miembro"
            verbose_name_plural = "Miembros"
            index_together = [
                    ["member","read_access"],
                    ["member","write_access"]
            ]

    def __unicode__(self):
            return u"%s %s role:%s read:%s write:%s" % (self.member,self.research,self.membership_role,self.read_access,self.write_access)

My modelform is:

class ResearchFormStepTwo(ModelForm):
    class Meta:
            model = Membership

    def clean_member(self):
            data = self.cleaned_data
            member=data.get("member")
            if not member.is_active:
                    raise forms.ValidationError("El usuario no esta activado")
    def __unicode__(self):
            return self.data

Answer 1

You need to return the member instance in .clean_member() when it's valid. The missing return is an implicit None, which then causes the ValueError since Membership.member isn't optional.

class ResearchFormStepTwo(ModelForm):
    class Meta:
        model = Membership

    def clean_member(self):
        data = self.cleaned_data
        member = data.get("member")
        if not member.is_active:
            raise forms.ValidationError("El usuario no esta activado")
        return member

    def __unicode__(self):
        return self.data

https://docs.djangoproject.com/en/1.5/ref/forms/validation/#cleaning-a-specific-field-attribute