How to check if an element is present in a Django queryset?

Question

Is it like a regular python set?

Suppose I have the following queryset

entry_set = Entry.objects.all()

How do I check if Entry Object e is present in entry_set?

Solution 1

You can use the following code:

if e in Entry.objects.all():
        #do something

Or the best approach:

if Entry.objects.filter(id=e.id).exists():
        #do something

Solution 2

The best approach, according to Django documentation: https://docs.djangoproject.com/en/2.1/ref/models/querysets/#exists

if Entry.objects.filter(id=item.id).exists():
    # Do something

But you can also do:

if item in Entry.objects.all():
    # Do something

Although this approach is the worser as possible. Because it will loop over the whole Queryset pulling elements from the database one by one, compared to the other approach that is almost everything done in the Database level.

If you have a list of ids or a Queryset others approaches would be use __in

Example with a Queryset:

query_ids = other_queryset.values_list('field_id', flat=True)
if Entry.objects.filter(id__in=query_ids).exists():
    # Do something

Or if you have a list of ids:

if Entry.objects.filter(id__in=[1, 2, 3, 4, 5]).exists():
    # Do something

Keep in mind that every time that you do len(queryset), item in queryset or list(queryset) you decrees heavily the performance of Django. I already see cases where by avoiding this practices We improved dozens of seconds in an application.

Solution 3

You can use in operator:

entry_set = Entry.objects.all()
if an_entry in entry_set:
    # The element present.

Comments

[Brian]

What if I have an Entry title instead of Entry object and want to check if that title exists in queryset?

[awesoon]

@Brian, you could do something like Entry.objects.filter(title='Your awesome title').exists()

[Lucas Wieloch]

Entry.objects.filter(id__in=[1, 2, 3, 4, 5]).exists() will return True if at least one element of the list is inside the queryset

[Alex Griffith]

using .exists() will prevent you from bringing in extra values if you can't filter off a unique key. It can also be used directly in the if statement, saving use of a variable. e.g. 'if Entry.objects.filter(id=e.id).exists(): ... do something here '

[lbris]

Now in Django 4.0 you can use contains() method instead, if you ave already fetched the Queryset : docs.djangoproject.com/fr/4.0/ref/models/querysets/#contains