Re: Can the new `Prefetch` solve my problem?

[aRkadeFR]

Thanks a lot for your answer :)

I will definitely use it from now on.

On 03/03/2015 06:59 AM, Simon Charette wrote:

Hi cool-RR,

The following should do:

filtered_chairs = Chair.objects.filter(some_other_lookup=whatever)
desks = Desk.objects.prefetch_related(
    PrefetchRelated('chairs', filtered_chairs, to_attr='filered_chairs'),

PrefetchRelated('nearby_chairs', filtered_chairs, to_attr='filtered_nearby_chairs'),

)

from itertools import chain
for desk in desks:
    for chair in chain(desk.filtered_chairs, desk.filtered_nearby_chairs):
        # ....

It will issue only three queries independently of the number of Chair or Desks you have.

Simon

Le mercredi 25 février 2015 15:05:50 UTC-5, cool-RR a écrit :

    Hi guys,

    I'm trying to solve a problem using the new `Prefetch` but I can't
    figure out how to use it.

    I have these models:

        class Desk(django.db.models.Model):
            pass
        class Chair(django.db.models.Model):
            desk = django.db.models.Foreignkey('Desk',
    related_name='chair',)
            nearby_desks = django.db.models.ManyToManyField(
                'Desk',
                blank=True,
            )

    I want to get a queryset for `Desk`, but it should also include a
    prefetched attribute `favorite_or_nearby_chairs`, whose value
    should be equal to:

        Chair.objects.filter(
            (django.db.models.Q(nearby_desks=desk) |
    django.db.models.Q(desk=desk)),
            some_other_lookup=whatever,
        )

    Is this possible with `Prefetch`? I couldn't figure out how to use
    the arguments.


    Thanks,
    Ram.

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