[Django]-How to eliminate inefficiency of django query in a loop?

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6👍

✅

You could use Subquery like in this answer https://stackoverflow.com/a/43883397/3627387 or with use of Prefetch https://stackoverflow.com/a/31237026/3627387

Here is one way to achieve this with Prefetch:

schedules_prefetch = Prefetch(
        'schedule_set',
        queryset=Schedule.objects.filter(user=user))
for question in questions_queryset.prefetch_related(schedules_prefetch):
    try:
        # using max here so it wouldn't do another DB hit
        schedule = max(question.schedule_set.all(),
                       key=lambda x: x.datetime_added)
    except ValueError:
        schedule = None

Here is an example using Subquery(it might not actually work, but will give you the general idea):

from django.db.models import OuterRef, Subquery
schedules = (Schedule.objects
             .filter(user=user, question=OuterRef('pk'))
             .order_by('datetime_added'))
questions_queryset = (questions_queryset
                    .annotate(latest_schedule=Subquery(schedules[:1])))
for question in questions_queryset:
    schedule = question.latest_schedule
👤Sardorbek Imomaliev

2👍

    # Get the question ids    
    question_ids = questions_queryset.values_list('id', flat=True)

    # get the beloved shedule
    schedule = Schedule.objects.filter(question__in=question_ids, user=user).latest(field_name='datetime_added')

    # You may opt for Schedule.objects.get() so as not to run into
    # the problem of multiple objects returned if all you need is strictly one schedule
👤Erick M

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