Django: Can't set ForeignKey value to None from admin

Question

I have a model Category which has a ForeignKey to a SimplePage model. null and blank are set to True. The problem is, when I edit a Category from the admin interface, I can't change the ForeignKey to --------- (Which looks like the admin's way of saying None.) The value can be None initially, and I can change it to an actual value through the admin, and I can change to another value, but once it has an actual value I can't change it back to None. (Through the admin, that is.)

Why is this?

UPDATE:

Here's the code for models.py:

from django.db import models
import tinymce.models
from photologue.models import Photo

from django.utils.translation import ugettext_lazy as _
import multilingual

class SimplePage(models.Model):

    slug = models.SlugField(
        _('Slug'),
        unique=True,
        help_text=_('''Unique identifier for URL. Only letters, digits, and -.\
e.g. history-oct-2000 or about''')
    )

    category = models.ForeignKey('Category',
                                 related_name='items_including_main_page',
                                 blank=True, null=True)

    class Translation(multilingual.Translation):
        title = models.CharField(_('Title'), max_length=42)
        content = tinymce.models.HTMLField(_('Content'), blank=True)

    class Admin:
        list_display = ('title',)
        search_fields = ('title', 'content')

    class Meta:
        verbose_name = _('Simple page')
        verbose_name_plural = _('Simple pages')

    __unicode__ = lambda self: self.title


class Category(models.Model):
    main_page = models.OneToOneField(
        SimplePage,
        related_name='_SimplePage__category_which_has_this_as_title', 
        blank=True,
        null=True)

    get_title = lambda self: self.main_page.title if self.main_page else u''

    get_items = lambda self: \
        self.items_including_main_page.exclude(id__exact=self.main_page.id)

    __unicode__ = lambda self: self.get_title() or u'Titleless Category'

    class Admin:
        pass

    class Meta:
        verbose_name = _('Category')
        verbose_name_plural = _('Categories')

And admin.py:

from sitehelpers.models import *
from django.contrib import admin
import multilingual

class SimplePageAdmin(multilingual.ModelAdmin):
    pass

admin.site.register(SimplePage, SimplePageAdmin)

admin.site.register(Category)

Answer 1

Maybe that's because you have OneToOne relationship defined also in category and you can't therefore break this relationship. Try to remove it and see, if you can set category in SimplePage to None.

Answer 2

gruszczy above is right, each side of o2o relation cannot be null after it was assigned, because django won't let you have orphan object in DB.