orm - Making queries using F() and timedelta at django

Question

I have the following model:

class Process(models.Model):
  title = models.Charfield(max_length=255)
  date_up = models.DateTimeField(auto_now_add=True)
  days_activation = models.PositiveSmallIntegerField(default=0)

Now I need to query for all Process objects that have expired, according to their value of days_activation.

I tried

from datetime import datetime, timedelta

Process.objects.filter(date_up__lte=datetime.now()-timedelta(days=F('days_activation')))

and received the following error message:

TypeError: unsupported type for timedelta days component: F

I can of course do it in Python:

filter (lambda x: x.date_up<=datetime.now() - timedelta(days=x.days_activation), 
        Process.objects.all ()), 

but I really need to produce a django.db.models.query.QuerySet.

Answer 1

You have to extend Aggregate. Do like below:

from django.db import models as DM

class BaseSQL(object):
    function = 'DATE_SUB'
    template = '%(function)s(NOW(), interval %(expressions)s day)'

class DurationAgr(BaseSQL, DM.Aggregate):
    def __init__(self, expression, **extra):
        super(DurationAgr, self).__init__(
            expression,
            output_field=DM.DateTimeField(),
            **extra
        )

Process.objects.filter(date_up__lte=DurationAgr('days_activation'))

Hopefully, It will work for you. :)