Question

In other words, why does this work:

if choice==Item.DO_ADD_CAT:
    for item in selected_items:
        for c in categories:
            item.categories.add(Category.objects.get(pk=c))
        item.save()
    messages.success(request,'Categories have been added.')

and this not:

if choice==Item.DO_REM_CAT:                        
    for item in selected_items:                            
        for c in categories:                                 
            item.categories.remove(Category.objects.get(pk=c))
        item.save()                            
    messages.success(request,'Categories have been removed.')

Isn’t this supposed to be working?

edit:
here is the Item model:

class Item(models.Model):
    #public/private state flags
    PRIVATE_STATUS=1
    PUBLIC_STATUS=2
    RELEASED_STATUS=3
    STATUS_CHOICES=((PRIVATE_STATUS ,'private'),
                    (PUBLIC_STATUS  ,'public' ),
                    (RELEASED_STATUS,'released'))
    status = models.IntegerField(choices=STATUS_CHOICES,
                                 default=PRIVATE_STATUS,
                                )
    objects = models.Manager()
    name=models.CharField(max_length=50)
    tags=TagField()
    count=models.IntegerField(blank=True)

    def get_user_path(self,filename):
        return '%s/%s/%s' %( self.creator_id,
                             datetime.date.today(),
                             filename)
    file=models.FileField(upload_to=get_user_path)

    creator = models.ForeignKey(User, related_name='creator')
    categories = models.ManyToManyField(Category,related_name="items",blank=True,null=True)  

and the Category Model:

class Category (models.Model):
    name = models.CharField(max_length=100)
    slug = models.SlugField()
    description = models.TextField(blank=True,help_text='Optional')

    def __unicode__(self):
        return self.name

    class Meta:
        verbose_name='Category'
        verbose_name_plural='Categories'

Answer 1

I guess i see the error… As in the django documentation tell us, you can not use remove on an foreignkey with a null=False definition… Related documentation

So prior to the logic of M2M relations, django set your m2m relation as null=False… So possibilities are:

1. change your m2m definition to null=True
2. use clear() to remove all elements…