class ActiveInactiveManager(models.Manager):
    """
    Used for models with active_date and inactive_date. Adds an "active" 
    filter to get only active items
    """
    def active(self):
        return self.filter(
            Q(active_date=None)   | Q(active_date__lte=now),
            Q(inactive_date=None) | Q(inactive_date__gt=now)
        )


# Usage in model:
class MyModel(models.Model):
    #...
    active_date = models.DateField()
    inactive_date = models.DateField()

    objects = ActiveInactiveManager()

"""
# usage example
>>> MyModel.objects.active()
>>> MyModel.objects.active().filter(category='whatever')
"""