Can not use celery delay for saved form: object is not JSON serializable

Question

Using Django 1.8, I'd like to trigger a delayed celery function after a form is saved in views

def new_topic(request, forum_id):
    form = TopicForm()
    uid = request.user.id
    if request.method == 'POST':
        tform = TopicForm(request.POST)
        if tform.is_valid():
            topic = tform.save(commit=False)
            topic.title = clean_title(tform.cleaned_data['title'])
            topic.description = clean_desc(tform.cleaned_data['description'])
            topic.save()
            notify_new_topic.delay( uid, topic) #<--problem here
            #rest of the views

But I get

EncodeError at /add/topic/
<Topic: Topic object> is not JSON serializable

I don't get any error if I remove delay from the celery task.

The task is:

@shared_task
def notify_new_topic(flwd_id, topic):
    title = topic.title
    link = topic.slug

    flwd= cached_user(flwd_id) #User.objects.get(id = flwd_id)
    print 'flwd is', flwd.username
    flwr_ids = FollowUser.objects.filter(followed=flwd).values('follower_id')
    flwrs = User.objects.filter(id__in= flwr_ids).values('id', 'username','email') 

    for f in flwrs:
        print 'flwr username:',  f['username']
        if notify_flwdp_applies(int(f['id'])):
            print 'notify flwdp applies'
            make_alerts_new_topic(flwd_id, f['id'], topic)
            print 'back from make_alerts_new_topic'

I'm wondering how can I debug/fix this?

Answer 1

Task's argument should be serializable (i.e. string, int, etc.). To fix error you can pass topic_id as argument and fetch topic object inside task method:

notify_new_topic.delay( uid, topic.id)

@shared_task
def notify_new_topic(flwd_id, topic_id):
    topic = Topic.objects.get(pk=topic_id)
    title = topic.title
    link = topic.slug

    flwd= cached_user(flwd_id) #User.objects.get(id = flwd_id)
    print 'flwd is', flwd.username
    flwr_ids = FollowUser.objects.filter(followed=flwd).values('follower_id')
    flwrs = User.objects.filter(id__in= flwr_ids).values('id', 'username','email') 

    for f in flwrs:
        print 'flwr username:',  f['username']
        if notify_flwdp_applies(int(f['id'])):
            print 'notify flwdp applies'
            make_alerts_new_topic(flwd_id, f['id'], topic)
            print 'back from make_alerts_new_topic'