Question

I want to check my Foo instance if it belongs to a category yet my category is a many to many attribute thus I get "'Foo' instance needs to have a primary key value before a many-to-many relationship can be used." once I try such:

class Category(models.Model):
    name = models.CharField(max_length=120)

class Movie (models.Model):
    poster = models.ImageField(blank=True)
    categories = models.ManyToManyField(Category) 

    def clean(self):
            try:
                self.categories.all().get(db_name="special")
                self.poster.url = u'/media/special_img.png'
            except Category.DoesNotExist:
                pass

Answer 1

You can’t check if Foo belongs to a category yet until you save it. But since it isn’t saved anyway, you can be sure that it doesn’t belong to a category yet.

One way to check wheter it has been saved already is by using if foo.pk. The pk is the primary key and it will be None if it hasn’t been saved yet.