Reevaluating a model-level query - django

Question

In brief: A model's method performs a query (returning the output of objects.filter()), but when the objects' values are changed in the database, the results of objects.filter() don't update until I bounce the server. How can I force the query to evaluate each time the method is called?
The details:
At the model level, I've defined a method to return all non-expired Announcement objects:
class AnnouncementManager(models.Manager):
# this is the method
def activeAnnouncements(self, expiry_time):
activeAnnouncements = self.filter(expires_at__gt=expiry_time).all()
return activeAnnouncements
class Announcement(models.Model):
...
expires_at = models.DateTimeField("Expires", null=True)
objects = AnnouncementManager()
I call this from a view with:
activeAnnouncements = Announcement.objects.activeAnnouncements()
However, when an Announcement object's data is updated in the database (e.g. expires_at is changed), the query still reflects the old data until the server is bounced. After reading http://docs.djangoproject.com/en/dev/ref/models/querysets/#when-querysets-are-evaluated, I tried to force the query to reevalute by updating the method as follows:
def activeAnnouncements(self, expiry_time):
# use boolean evaluation to force reevaluation of queryset
if self.filter(expires_at__gt=expires):
pass
activeAnnouncements = self.filter(expires_at__gt=expiry_time).all()
return activeAnnouncements
This had no effect.
Thanks for your help!
Update:
Can you please show the full code of where you are calling it?
This is the view which calls it:
#never_cache
def front_page(request):
'''
Displays the current announcements
'''
announcements = ''
activeAnnouncements = Announcement.objects.activeAnnouncements().order_by('-id')
if not request.user.get_profile().admin:
hide_before = request.user.get_profile().suppress_messages_before
if hide_before is not None:
activeAnnouncements = activeAnnouncements.filter(created_at__gt=hide_before)
if activeAnnouncements.count() > 0:
announcements = activeAnnouncements
else:
announcements = ""
return render_to(
request
, "frontpage.html"
, {
'announcements' : announcements
})
And here's the full version of the Announcement and AnnouncementManager models (excerpted above):
class AnnouncementManager(models.Manager):
# Get all active announcements (i.e. ones that have not yet expired)
def activeAnnouncements(self, expires=datetime.datetime.now()):
activeAnnouncements = self.filter(expires_at__gt=expires).all()
return activeAnnouncements
class Announcement(models.Model):
text = models.TextField()
subject = models.CharField(max_length=100)
expires_at = models.DateTimeField("Expires", null=True)
created_at = models.DateTimeField("Creation Time", auto_now_add=True)
created_by = models.ForeignKey(User, related_name="created_announcements")
updated_at = models.DateTimeField("Update Time", auto_now=True)
updated_by = models.ForeignKey(User, related_name="updated_announcements")
objects = AnnouncementManager()
def __unicode__(self):
return self.subject

Answer 1

Aha. The full version of the Manager method has a big difference from the one you originally posted, and it's there that the trouble is.
def activeAnnouncements(self, expires=datetime.datetime.now()):
This is one of the biggest Python gotchas: default function parameters are evaluated when the function is defined, not when it is called. So the default value for expiry will be set to whenever the server process was first started. Read the effbot's explanation of the problem. (Note it's a Python problem, not anything to do with Django querysets.)
Instead, do this:
def activeAnnouncements(self, expires=None):
if expires is None:
expires = datetime.datetime.now()
activeAnnouncements = self.filter(expires_at__gt=expires).all()
return activeAnnouncements

Answer 2

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