Map (apply function) Django QuerySet

user_3286487 0
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Is there a mechanism to map Django QuerySet items not triggering its evaluation?

I am wondering about something like Python map. A function that uses a function to apply it over the QuerySet, but keeping the lazy evaluation.

For example, using models from Django documentation example, is there something like? (not real code):

>>> Question.objects.all().map(lambda q: q.pub_date + timedelta(hours=1))

which keeps the lazy evaluation?

lazy-evaluation django
3 Answers

Just working through something like this myself. I think the best way to do it is to use a python list comprehension.

[q.pub_date + timedelta(hours=1) for q in Question.objects.all()]

Then Django takes care of optimizing this as it would any other query.

user_31174090

You can use annotate for this purpose, for example

from datetime import timedelta
from django.db.models import F, ExpressionWrapper, DateTimeField

Question.objects.annotate(
    new_pub_date=ExpressionWrapper(
       F('pub_date') + timedelta(hours=1), 
       output_field=DateTimeField()
    )
)

For something a little bit more complex than this example, you can use Func, Case, When

user_78690750

You can use values_list to get any column you like:

Question.objects.values_list('pub_date')

This is simpler than anything you can cook up yourself.

user_737500
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