Django: filtering queryset by 'field__isnull=True' or 'field=None'?

Question

I have to filter a queryset by a dynamic value (which can be None): may I simply write:

filtered_queryset = queryset.filter(field=value)

or shall I check for None:

if value is None:
    filtered_queryset = queryset.filter(field__isnull=True)
else:
    filtered_queryset = queryset.filter(field=value)

Does the behaviour depend on the particular DBMS?

Solution 1

The ORM will handle None (cast it to NULL) for you and return a QuerySet object, so unless you need to catch None input the first example is fine.

>>> User.objects.filter(username=None)
[]
>>> type(_)
<class 'django.db.models.query.QuerySet'>
>>> str(User.objects.filter(username=None).query)
SELECT "auth_user"."id", "auth_user"."username", "auth_user"."first_name", "auth_user"."last_name", "auth_user"."email", "auth_user"."password", "auth_user"."is_staff", "auth_user"."is_active", "auth_user"."is_superuser", "auth_user"."last_login", "auth_user"."date_joined" FROM "auth_user" WHERE "auth_user"."username" IS NULL

Solution 2

I prefer the second solution which is handled better

Update

if value is None:
    filtered_queryset = queryset.filter(field__isnull=True)
    # Do some proessing with  filtered_queryset object with None values
else:
    filtered_queryset = queryset.filter(field=value)
    # Do some proessing with  filtered_queryset object with not NULL values

Query set can handle Null values..Based on this User.objects.filter(username=None) this would fetch only values where username=NULL

Comments

[Ngenator]

+1 I was unaware it handled None like that, very useful to know.

[Rajeev]

The only reason which suggested your second condition was if u wanted to handle None conditions and do some processing on it then u could have used the second solution.Please see the updated answer..

[Don]

Thanks. One last question: when you say "this would fetch only values where username=NULL", you mean that Django would translate into "username IS NULL" in case it is required by the DB backend?

[Rajeev]

For example 1)queryset = MyModel.objects.all() 2)print queryset.query 3)SELECT "app_mymodel"."id", ... FROM "app_mymodel"

[Akhorus]

I'm using Django 1.7 and both cases generate the same query: In [19]: str(User.objects.filter(username__isnull=True).query) == str(User.objects.filter(username=None).query) Out[19]: True So, I suppose you can use either.