Sample Django model of an expiring message

models.py

from datetime import timedelta
from django.conf import settings
from django.db import models
from django.utils import timezone


class ActiveMessagesManager(models.Manager):
    use_for_related_fields = True

    def get_queryset(self):
        return super(ActiveMessagesManager, self).get_queryset().filter(expiration_dt__gt=timezone.now())


def default_expiration_dt():
    return timezone.now() + timedelta(hours=3)


class Message(models.Model):

    sender = models.ForeignKey(settings.AUTH_USER_MODEL, related_name='messages_sent')
    receiver = models.ForeignKey(settings.AUTH_USER_MODEL, related_name='messages_received')
    text = models.TextField()
    expiration_dt = models.DateTimeField(default=default_expiration_dt)

    active = ActiveMessagesManager()