Hello Yo'av,
You'll want to use subqueries for this[0].
from django.db.models import OuterRef, Subquery
children =
Child.objects.filter(adult=OuterRef('pk')).order_by('dob').values('dob')
Adult.objects.annotate(
oldest_son_dob=Subquery(children.filter(gender='M')[:1]),
oldest_daughter_dob=Subquery(children.filter(gender='F')[:1]),
)
Note that I haven't tried the above code myself so it might required
adjustments.
Cheers,
Simon
[0]
https://docs.djangoproject.com/en/1.11/ref/models/expressions/#subquery-expressions
Le dimanche 21 mai 2017 10:41:44 UTC-4, Yo'av Moshe a écrit :
>
> Hey Djangoists!
> I can't get my head around this and I'm not sure if it's even possible.
>
> Let's say I have an "Adult" object and a "Child" object. Every Child
> belongs to an Adult, and has a gender which is either "M" or "F", and also
> a "dob" field with their date of birth. I want to get a list of all adults
> annotated with the dob of their oldest son, and the dob of their oldest
> daughter.
>
> How am I to do this?
>
> I tried something like this:
> Adult.objects.annotate(
> oldest_son_dob=Case(
> When(children__gender="M", then=F('children__dob')),
> default=None,
> output_field=DateField(),
> )
> )
>
> # ... same for daughter
>
>
> but I'm not sure where to tell Django that I only want it to pick the
> oldest child, and so right now it duplicates the adult object for every
> child it has.
>
> Does Django support this kind of query?
>
> I'm using PosgresSQL FWIW.
>
> Thank you so much
>
> Yo'av
>
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