How to get hold of a session after getting signaled?

[Tobias Dacoir]

I'm using django-badges in my project which can emit a signal when a user is awarded a badge. I was able to implemented a receiver function, however the output is the following:

signals.py (from badges):

import django.dispatch

badge_awarded = django.dispatch.Signal(providing_args=['user', 'badge'])

my receiver function test:

from badges.signals import badge_awarded

@receiver(badge_awarded)
def do_something_after_badge_awarded(sender, user, badge, **kwargs):
    print "Badge Signal received"
    print sender # <play.meta_badges.neueBadge object at 0x109733d90>
    print user # user object
    print badge # badge object
    print kwargs

and the output was:

Badge Signal received
<play.meta_badges.neueBadge object at 0x109733d90>
player1
Test Badge 2
{'signal': <django.dispatch.dispatcher.Signal object at 0x1096da4d0>}

I want to add a message to the request, to notify or show the user that he earned a badge. Is it possible someone get hold of the request? I can freely modify the original source code from django-badges. Currently it's invoked by post_save() on a model instance. So I doubt I can get the request from there.

I found a solution to add another 3rd party app which is called django-notification, but I don't want to add too many apps to my project. Another solution might be to just set some variable in the database and in my view to query this variable - however that will also cost performance and two database hits.

[Vijay Khemlani]

As far as I can tell on the project source, the only place the "badge_awarded" signal is triggered is in the "award_to" method in the Badge class, which does not handle a request object.

If you are calling something like "badge.award_to(user)" in one of your views, then you can modify the request response immediately after, without using the signal.

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[Tobias Dacoir]

I can't just call badge.award_to(user) because I need to wait for the check logic to verify if the user should receive a badge or not.
My workaround is that I created a new table that inserts badge and user foreign keys and before the view is rendered I will check this table for any entries for that user. If there are, I will copy the data to a message and delete the object from the database again.

It's rather ugly due to the many database hits which I would like to avoid but I can't think of anything else at the moment. Apparently (using google) there is no way to get hold of the request in a post_save() signal - which I need to check all achievements.

The only real solution would be to not use this post_save() signal but manually do the checking for achievements before my view is rendered, which would require some work on the original source code. I will look into it though next week.

[Stephen J. Butler]

You still could store the do_something_after_badge_awarded result in a
database table, but cache the lookups from the view for something
short, like 5 minutes. How quickly after a badge is awarded do people
need to be notified?

Also, you could use a memcached library to store/fetch the awards by
user ID. Should be pretty quick. That assumes of course that it's OK
to lose award notifications if memcached restarts. Would look
something like:

# store
mc.append(key_for_user, "{0} ".format(award_obj.pk))

# ...

# get. use a lockless cas() algo
award_objs = None
while award_objs is None:
award_ids, cas_token = mc.gets(key_for_user)
if not award_ids:
# No awards waiting for us
award_objs = []
elif mc.cas(key_for_user, "", cas_token):
# Awards didn't change on us, continue
award_objs = Awards.objects.filter(pk__in=award_ids.split())

Or you could do something with the Django cache framework. Or you
could use local files and serialization.

But I do think you should test your assumption that the DB check for
awarded awards will be too expensive. Especially if cached. Most apps
make a ton of DB calls anyway, and this one will mostly be empty.

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[Babatunde Akinyanmi]

+1. You need to measure rather than assume.

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