#10227: Support a related_default on OneToOne fields
-------------------------------------+-------------------------------------
Reporter: rvdrijst | Owner: nobody
Type: New feature | Status: new
Component: Database layer | Version: master
(models, ORM) | Resolution:
Severity: Normal | Triage Stage: Accepted
Keywords: onetoone related | Needs documentation: 0
expection null | Patch needs improvement: 0
Has patch: 0 | UI/UX: 0
Needs tests: 0 |
Easy pickings: 0 |
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Comment (by akaariai):
The example of related_default usage is exactly what I am afraid. That
code contains a serious pitfall: the user.profile isn't saved, and it will
not be associated to the user if you don't save it. And, you do get that
default value even for saved user instances.
There is a big difference to how foreign key default argument works:
{{{
class A(models.Model):
pass
def get_a():
return A.objects.get_or_create(pk=1)[0]
class B(models.Model):
a = models.ForeignKey(A, null=True, default=get_a)
b = B()
print b.a
b = B(a=None)
print b.a
b = B(a=None)
b.save()
b = B.objects.get()
print b.a
}}}
The output is:
{{{
A object
None
None
}}}
My understanding is that the "related_default" would return "A object" for
each case. This is broken - what you get from b.a doesn't reflect the
database state.
The related_none could work. Here I am mainly worried of not knowing what
happens when I write:
{{{
p = request.user.profile
}}}
does it raise `DoesNotExist` if the profile doesn't exists, or will p just
be None? However, I do not feel too strongly about this case.
The good thing about b.nget('a') is that when you see the code, you
immediately know what the code does. You are explicitly asking for a value
that can be None value, so you will be prepared to handle the None case.
This is relatively simple to do in user code, so there is no strong reason
to have this convenience method in Django core.
--
Ticket URL: <https://code.djangoproject.com/ticket/10227#comment:26>
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