How to filter using an expression in Django

Question

I'd like to filter that implements this "pseudocode":

   Post.objects.filter(Post.timestamp + Post.duration > datetime.datetime.now())

I also would like to wrap this in a Django management command.

Any help would be great!

Answer 1

Filter

Not sure how your fields look but here's a hint:

Let's compose an F expression like this F('timestamp') - F('duration'), and annotate our query with it:

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

Post.objects.annotate(
        timestamp_minus_duration=ExpressionWrapper(
            F('timestamp') + F('duration'),
            output_field=DateTimeField()
        )
    )

Now you can filter with that annotated field

   Post.objects.annotate(
        timestamp_minus_duration=ExpressionWrapper(
            F('timestamp') + F('duration'),
            output_field=DateTimeField()
        )
    ).filter(
        timestamp_minus_duration__gt=datetime.datetime.now()
    )

ref: https://docs.djangoproject.com/en/1.9/topics/db/queries/#using-f-expressions-in-filters

ref: https://docs.djangoproject.com/es/1.9/ref/models/expressions/#using-f-with-annotations

ref: https://docs.djangoproject.com/es/1.9/topics/db/aggregation/#filtering-on-annotations

Management Command

Just put the code in the handle() method of the command

# yourapp/management/commands/deletepost.py

from django.core.management.base import BaseCommand, CommandError
from yourapp.models import Post

class Command(BaseCommand):
    help = 'Describe your cmd here'

    def handle(self, *args, **options):
           Post.objects.annotate(...).filter(...).delete()

More details: https://docs.djangoproject.com/en/1.9/howto/custom-management-commands/