Django: accessing ManyToManyField objects after the save [Solved]

Question

This is baffling me... When I save my model, the book objects are unchanged. But if I open the invoice and save it again, the changes are made. What am I doing wrong?

class Invoice(models.Model):
    ...
    books = models.ManyToManyField(Book,blank=True,null=True)
    ...

    def save(self, *args, **kwargs):
        super(Invoice, self).save(*args, **kwargs)
        for book in self.books.all():
            book.quantity -= 1
            if book.quantity == 0:
                book.sold = True;
            book.save()

Edit: I've tried using the post_save signal, but it works the same way. No changes on the first save, changes saved the second time.

Update: Seems to be solved with this code:

class InvoiceAdmin(admin.ModelAdmin):
    ...

    def save_model(self, request, obj, form, change):
        obj.save()
        for bk in form.cleaned_data['books']:
            book = Book.objects.get(pk=bk.id)
            book.quantity -= 1
            if book.quantity == 0:
                book.sold = True;
            book.save()

Answer 1

That's because m2m relation are saved after your model save, in order to obtain PK of parent object. In your case, second save works as expected because model already has PK and associated books from first save (it's done in a signal).

I haven't found the solution yet, best bet is to do your changes in admin view, i guess.