Hi Chris,
Without wanting to preclude discussion of this kind of functionality ending
up in Django itself, which I think would be great - I created
https://github.com/alexhill/django-relativity to meet this kind of need.
In your example, it would look a like this:
from relativity import Relationship, L
class A(models.Model):
...
b = models.ForeignKey(B)
c = models.ForeignKey(C)
d = Relationship(D, predicate=Q(a_id=L("a_id"), b_id=L("b_id"))
Now the field `d` is available on all instances of A, in ORM query syntax,
etc. As far as possible, it should work like a native Django relation.
The predicate is an arbitrary Q against the related model which can include
L objects, which refer to fields in the L for local model.
Let me know if this helps!
Cheers,
Alex
On Tue, Jun 29, 2021 at 11:14 PM Chris Le Sueur <thefishf...@gmail.com>
wrote:
> Hi all,
>
> In https://code.djangoproject.com/ticket/30653 a feature request was made
> to support annotating instances with an entire queryset. This was closed
> wontfix because you can do this with the prefetch_related mechanism.
>
> However, prefetch_related is very limited due to the design intention to
> support prefetching relationships defined in the ORM. My use case is to
> annotate each instance fetched in a queryset with a related queryset, where
> which objects are annotated depends on two fields of the original instance.
> Consider models A, B, C and D, where both A and D have foreign keys to both
> B and C, and you want to annotate a queryset of As with a collection of Ds
> which share the same pair of related B and C. (This is exactly the use case
> I have in mind, but of course it could be quite general.)
>
> Prefetch objects cannot be used because, firstly, a Prefetch on b__d_set
> (for example) will annotate the B models: there is no way to collect the D
> instances onto the A models; and secondly because there is no way to filter
> or join on the second field.
>
> prefetch_related is making some assumptions about the correct way to
> perform queries because of what it's there to do; a complete method for
> annotating querysets onto things would perhaps not want to query the
> related table via a "WHERE pk IN (...)" clause but instead query the root
> table's pk and JOIN on the subsequent tables - in the above scenario this
> would be very simple (... JOIN d ON (a.b_id = d.b_id AND a.c_id = d.c_id)).
> This avoids having to faff around getting ids for all the intermediate
> models in python and pass them back to the database (though presumably
> there's a reason for doing that in prefetch_related; maybe it would make
> sense here too.)
>
> The big issue for me is that there is not a good workaround for this at
> the moment. Suppose you frequently need access to things of this type (and
> need the performance gained by querying everything in one go) then you are
> forever querying tables directly and assembling dictionaries - instead of
> using the ORM to do this for you. If frequently need to do this for *the
> same* query, you can't really do it in a custom queryset without overriding
> _fetch_all, assembling the dictionary there and gluing collections onto
> instances there, which doesn't feel like a good thing to do.
>
> So I would suggest reopening the linked issue!
>
> Chris
>
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