Django: Make items in ManyToMany field unavailable once it is "used"?

Question

if this is my models.py:

class Category(models.Model):
    name = models.CharField(max_length=500)

    slug = models.SlugField(unique=True)

    def __unicode__(self):
        return self.name            

    def get_absolute_url(self):
        return "%s" % self.slug 


class Item(models.Model):
    name = models.CharField(max_length=500)

    slug = models.SlugField(unique=True)

    category = models.ManyToManyField(Category)     

    def __unicode__(self):
        return self.name            

    def get_absolute_url(self):
        return "%s" % self.slug 

My desired functionality is to have any Category that has already had a an Item assigned to it to not be available for any other Items. For example, if I have these Categories: { Animal, Vegetable, Mineral }, and i assign the Item "Rock" to the Category "Mineral", when I add the next Item, "Mineral" will be unavailable from the list in admin. Hopefully that makes sense, and thank you for your time.

Answer 1

You could specify an "available" field, something like this. And then just show "availables" to the user.

class Category(models.Model):
    name = models.CharField(max_length=500)
    slug = models.SlugField(unique=True)
    available = models.BooleanField(default=True)

    def __unicode__(self):
        return self.name            

    def get_absolute_url(self):
        return "%s" % self.slug 

    def save(self, force_insert=False, force_update=False, using=None):
        if self.item_set.all():
            self.available = False
        super(Category, self).save()

Answer 2

If you only want each Category to be associated with a single Item, then you haven't got a ManyToMany relationship at all. You should restructure your models to make it a ForeignKey from Category to Item.