On Wednesday 03 February 2010 20:21:49 kanu wrote:
> My Ticket http://code.djangoproject.com/ticket/12760 Titled :
> "related models with Foreignkey.null=True must not get deleted on
> delete of their relation" was marked as dublicate today.
>
> I don't aggree that this is a dublicate. i filed this as a bug not
> a feature request.
This 'bug' has been reported various times, and it depends on your
point of view whether it is a bug or not. There are certainly cases
where the opposite would be a bug. Consider, for instance, this
model:
class Employee(Model):
boss = ForeignKey('self', null=True)
The semantics of this model might include the idea that employees with
boss == NULL are directors. Now, if we delete employee e1 who happens
to be the boss of employee e2, and just null out e2.boss, we have
promoted e2 to being a director, which is not intended.
So this is a feature request, and on your bug, I linked to the most
recent, open bug covering it.
> I really think that it was intendet to not delete those relations.
> e.g. the comments in CollectedObjects.add say :
> # Nullable relationships can be ignored -- they are nulled out
> before # deleting, and therefore do not affect the order in which
> objects # have to be deleted.
>
> Even if it is not a bug the code appears a little confusing.
> Why is a relation updated and afterwards deleted ?
Karen is right, it allows mutually related data to be deleted without
causing an integrity error in the absence of transactions:
class A(Model):
b = ForeignKey(B)
class B(Model):
a = ForeignKey(A, null=True)
By noticing the null=True, we can null out B.a, then delete the A
object, then the B object, all without requiring a transaction.
CollectedObjects uses this logic to decide that the correct order of
deletion is A,B. Another part of the code actually does the nulling
out and deleting.
Hope that explains it, sorry if the message was rather brief, but this
has come up several times before and we can't explain it every time.
Cheers,
Luke
--
"Where a person wishes to attract, they should always be ignorant."
(Jane Austen)
Luke Plant || http://lukeplant.me.uk/
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