Feature request: ForeignKey through parameter
Roald de Vries
I quite often reference foreign keys of foreign keys of foreign
keys... Wouldn't it be nice to have a 'through'-parameter for
ForeignKey's?
class A(Model):
b = ForeignKey('B')
c = ForeignKey('C', through='B', related_name='a_set')
class B(Model):
c = ForeignKey('C')
The advantages of a field A.c wrt a property A.c:
- it creates a reverse relation, so C.a_set becomes available
- c becomes queryable: A.objects.filter(c=x)
For 'through'-ing one more class you could use:
class Z(Model):
a = ForeignKey('A')
c = ForeignKey('C', through='A') # works because A.c exists
or:
class Z(Model):
a = ForeignKey('A')
c = ForeignKey('C', through='A__B') # works regardless of
whether A.c exists
I'm curious if other people would be interested in such a feature, or
maybe if something alike or better already exists, and whether there
are downsides that I miss.
Cheers, Roald
Javier Guerra Giraldez
> I quite often reference foreign keys of foreign keys of foreign keys...
> Wouldn't it be nice to have a 'through'-parameter for ForeignKey's?
>
>
> class A(Model):
> b = ForeignKey('B')
> c = ForeignKey('C', through='B', related_name='a_set')
>
> class B(Model):
> c = ForeignKey('C')
i'd love such a feature too, but i think a better syntax could be
something like:
class A(Model):
b = ForeignKey('B')
c = ForeignKey('B__c', related_name='a_set')
class B(Model):
c = ForeignKey('C')
where the second part of the reference is the name of the field ('c'
in this example), not the model class ('C')
--
Javier
Roald de Vries
On first sight, I think I agree with you that the syntax is cleaner
like this, but I would choose for the through-parameter because it's
more consistent with the use of the through-parameter for
ManyToManyField.
Javier Guerra Giraldez
> On first sight, I think I agree with you that the syntax is cleaner like
> this, but I would choose for the through-parameter because it's more
> consistent with the use of the through-parameter for ManyToManyField.
but what if B has more than one ForeignKey('C') fields?
--
Javier
Adrian Holovaty
> I quite often reference foreign keys of foreign keys of foreign keys...
> Wouldn't it be nice to have a 'through'-parameter for ForeignKey's?
>
> class A(Model):
> b = ForeignKey('B')
> c = ForeignKey('C', through='B', related_name='a_set')
>
> class B(Model):
> c = ForeignKey('C')
>
>
> The advantages of a field A.c wrt a property A.c:
> - it creates a reverse relation, so C.a_set becomes available
> - c becomes queryable: A.objects.filter(c=x)
Hi Roald,
Thanks for suggesting this. I don't think it's worth implementing,
though, because you can already query across multiple foreign-key
relationships like this:
A.objects.filter(b__c__exact=x)
As for making C.a_set available, I'd suggest just doing
A.objects.filter(b__c__exact=c_instance) in your view, and that'll get
the job done.
Adrian