On 12.02.2010 14:01, Tom Evans wrote: > I think the easiest way of understanding the behaviour is that > specifying a field like this: > > owner = models.ForeignKey(Owner) > > specifies a contract. The contract says that when you access this > attribute on an instance, it will only return an instance of an Owner > object or raise an DoesNotExist exception. > > Specifying a field like this: > > owner = models.ForeignKey(Owner,blank=True,null=True) > > specifies a very different contract. This contract says that accessing > this attribute will return either an instance of an Owner object, or > None if no owner has been specified. > > Both contracts are fully described in the documentation and fulfilled > by the ORM, so clearly this is no bug. Both flavours are available for > you to use, you simply have to decide what sort of contract you want > that attribute to have - if you expect instance.owner to return None, > then clearly you should be using a nullable foreign key.
Wow! This formulation is really clear. Thank you, Tom. My summary: Django's behaviour is un-pythonic and not the most elegant one, but it is possible to get used to it, even to rely on it, and it certainly won't change. I didn't find such a good explanation in the docs, though. IMHO it should be here: http://docs.djangoproject.com/en/dev/topics/db/queries/#forward I suggest to add a paragraph about the surprising DoesNotExist there. Luc -- You received this message because you are subscribed to the Google Groups "Django developers" group. To post to this group, send email to django-develop...@googlegroups.com. To unsubscribe from this group, send email to django-developers+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/django-developers?hl=en.
