django-queryset – Page 6

Django update queryset with annotation

May 21, 2023 by Tarik

For Django 1.11+ you can use Subquery: from django.db.models import OuterRef, Subquery, Sum Relation.objects.update( rating=Subquery( Relation.objects.filter( id=OuterRef(‘id’) ).annotate( total_rating=Sum(‘sign_relations__rating’) ).values(‘total_rating’)[:1] ) ) This code produce the same SQL code proposed by Tomasz Jakub Rup but with no use of RawSQL expression. The Django documentation warns against the use of RawSQL due to the possibility of … Read more

django select_related – when to use it

May 17, 2023 by Tarik

You are actually asking two different questions: 1. does using select_related actually creates performance overhead? You should see documentation about Django Query Cache: Understand QuerySet evaluation To avoid performance problems, it is important to understand: that QuerySets are lazy. when they are evaluated. how the data is held in memory. So in summary, Django caches … Read more

OR operator in Django model queries

May 14, 2023 by Tarik

Q objects from django.db.models import Q Contact.objects.filter(Q(last_name__icontains=request.POST[‘query’]) | Q(first_name__icontains=request.POST[‘query’]))

Django excluding one queryset from another

May 12, 2023 by Tarik

blocked_delivery_times = BlockedDeliveryTime.objects.filter(delivery_date=delivery_date) \ .values(‘delivery‌_time’) delivery_times = DeliveryTime.objects.exclude(id__in=blocked_delivery_times)

Django Count() in multiple annotations

May 10, 2023 by Tarik

I think Count(‘topics’, distinct=True) should do the right thing. That will use COUNT(DISTINCT topic.id) instead of COUNT(topic.id) to avoid duplicates. User.objects.filter( username_startswith=”ab”).annotate( posts=Count(‘post’, distinct=True)).annotate( topics=Count(‘topic’, distinct=True)).values_list( “username”,”posts”, “topics”)

“_set” in a queryset object in Django

May 1, 2023 by Tarik

What you are seeing is a reverse related object lookup. In your example: class Blog(models.Model): pass class Entry(..): blog = Blog(..) Now, given object e of type Entry, you would do e.blog to access the related object Blog – which is a forward relation. The _set is a reverse lookup class variable django puts in … Read more

Get the index of an element in a queryset

April 30, 2023 by Tarik

If you’re already iterating over the queryset and just want to know the index of the element you’re currently on, the compact and probably the most efficient solution is: for index, item in enumerate(your_queryset): … However, don’t use this if you have a queryset and an object obtained by some unrelated means, and want to … Read more

prefetch_related for multiple Levels

April 26, 2023 by Tarik

Since Django 1.7, instances of django.db.models.Prefetch class can be used as an argument of .prefetch_related. Prefetch object constructor has a queryset argument that allows to specify nested multiple levels prefetches like that: Project.objects.filter( is_main_section=True ).select_related( ‘project_group’ ).prefetch_related( Prefetch( ‘project_group__project_set’, queryset=Project.objects.prefetch_related( Prefetch( ‘projectmember_set’, to_attr=”projectmember_list” ) ), to_attr=”project_list” ) ) It is stored into attributes with _list … Read more

Django query case-insensitive list match

April 26, 2023 by Tarik

Unfortunatley, there are no __iin field lookup. But there is a iregex that might be useful, like so: result = Name.objects.filter(name__iregex=r'(name1|name2|name3)’) or even: a = [‘name1’, ‘name2’, ‘name3’] result = Name.objects.filter(name__iregex=r'(‘ + ‘|’.join(a) + ‘)’) Note that if a can contain characters that are special in a regex, you need to escape them properly. NEWS: … Read more