Adding a field to queryset in django

Question

I have a model whose fields are datetimefields start_time and end_time I want to display on the API- start_time, end_time and the difference between them However, I don't want to use for or while loop to achieve this because it is too slow How can I get the difference on the API without looping

I don't get why looping is too slow. Django will loop as well since it retrieves the data from the database, and will do looping to "pack" the data in model objects. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented May 12, 2020 at 18:00
Have you tried something like this, ModelKlass.objects.annotate(diff=F('field_1')-F('field_2')) ? — JPG
– JPG, Commented May 12, 2020 at 18:01
@ArakkalAbu: the problem is that this is that Django's "type system" does not reason this to be a DurationField. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented May 12, 2020 at 18:06

willeM_ Van Onsem · Accepted Answer · 2020-05-12 18:05:24Z

1

You can annotate the queryset with the difference, and then use an ExpressionWrapper to convert this to a DurationField, and thus obtain a timedelta objects:

from django.db.models import DurationField, ExpressionWrapper, F

MyModel.objects.annotate(
    time_diff=ExpressionWrapper(
        F('end_time')-F('start_time'),
        output_field=DurationField()
    )
)

The MyModels that arise from this queryset will thus have an extra attribute time_diff that is a timedelta field containing the difference between end_time, and start_time.

answered May 12, 2020 at 18:05

willeM_ Van Onsem

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