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

Thanks Erik. Will try out the solution you mentioned.

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
    .filter(date_created=Max('a__b__date_created'))
    .annotate(Max('a__b__date_created'))
    .filter(text='ABCD')
    .select_related('a')
]

which you can rewrite as:

A.objects.filter(
    b__in=B.objects
        .filter(date_created=Max('a__b__date_created'))
        .annotate(Max('a__b__date_created'))
        .filter(text='ABCD')
)

Erik

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