Tuesday, August 30, 2016

Re: A tricky query in one to many relationship - atleast for me:)

Hi Erik,

I tried your solution but there are some issues:

.filter(date_created=Max('a__b__date_created'))  - this is throwing error saying not proper use of group function.

If I remove the above, the result isn't correct where when I go through each 'a' in the result, associated latest B.text isn't always 'ABCD' - if there are multiple instances of B associated with an instance of A and one of the instances of B has text='ABCD' (might not be the latest date_created), that instance of B is also there in the query result.


On Tuesday, August 30, 2016 at 3:59:20 PM UTC+5:30, Erik Cederstrand wrote:

> Den 30. aug. 2016 kl. 11.20 skrev Erik Cederstrand <erik+...@cederstrand.dk>:
> I'm not even sure that's possible to express in SQL, but it would probably be quite convoluted if it is. Here's an easier-to-understand solution:
> res = set()
> for b in B.objects.all().select_related('a').annotate(Max('date_created')):
>    if b.date_created != b.date_created__max:
>        continue
>    if b.text != 'ABCD':
>        continue
>    res.add(a)

I did some more experimenting. I think this actually does what you want:

res = [
    b.a for b in B.objects

which you can rewrite as:



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