gistfile1.py

class Book(models.Model):
    isbn = models.IntegerField(max_length=13)
    title = models.CharField(max_length=100)
    pub_date = models.IntegerField(max_length=4, choices=YEARS)

    authors = models.ManyToManyField(Author, related_name="books")
    categories = models.ManyToManyField(Category)

    def __unicode__(self):
        return self.title

class BookOnLoan(models.Model):

    class Meta:
        verbose_name_plural = 'Loaned Books'

    user = models.ForeignKey(User)
    # I want to limit choices to only Books that haven't been loaned out yet
    # I need Books that have date_returned == None
    # My problem is that I don't know how to filter for that from Book objects
    # Thanks for looking!
    book = models.ForeignKey(Book, limit_choices_to = ())

    date_issued = models.DateField(auto_now_add=True)
    date_due = models.DateField(default= date.today() + DAYS_ON_LOAN)
    date_returned = models.DateField(blank=True, null=True)

    def save(self, *args, **kwargs):
        super(BookOnLoan, self).save(*args, **kwargs)

    def __unicode__(self):
        return self.book.__unicode__() + " / " + self.user.__unicode__() + " / " + str(self.date_due)