Django - Page not fully loaded after exception in form - django

Question

In my django project, I have a view that displays details of an event. In that view there is a link to another view that contains the form to register people to that event.
If the maximum number of participants is reached and someone still tries to register, my view throws an exception. And returns from the formular-site back to the event-site. But the event-site is not fully loaded, I think the queries are no performed.
I don't know how to write the return function.
Additionally, is there a syntax error in my exception? When I create new instances of registration in the shell, I can save as many as I want.
def validate_category_full(category_id):
cat = Category.objects.get(id=category_id)
regs = cat.registration_set.all()
if len(regs) >= cat.max_people:
raise ValidationError('Category is already full.')
def registration(request, category_id, event_id):
"""Add a new registration to a category."""
cat = Category.objects.get(id=category_id)
myevent = Event.objects.get(id=event_id)
if request.method != 'POST':
form = RegistrationForm()
else:
form = RegistrationForm(request.POST)
try:
validate_category_full(cat.id)
except(ValidationError):
return render(request, 'events/event.html',
{'myevent': myevent,
'error_message': 'Event is full.',})
else:
if form.is_valid():
reg = form.save()
return HttpResponseRedirect(reverse('events:category',
args=(cat.id,)))
context = {'form': form, 'cat': cat, 'myevent': myevent}
return render(request, 'events/registration.html', context)`

Answer 1

Mostly a wild guess since I don't know what you mean by "the event-site is not fully loaded" (and assuming "event-site" and "formular-site" really mean "event page" and "formular page"):
First: you shouldn't render the template from another view but redirect to that view instead. If you want to display a message to the user (the "event is full" message in this case) when he gets redirected back to the other view, use the messages framework:
from django.contrib import messages
def registration(request, category_id, event_id):
# snip a lot of code
try:
validate_category_full(cat.id)
except(ValidationError):
messages.error(request, 'Event is full.')
return redirect(your_event_view_name, whatever_args_it_takes)
A few notes why we're at it:
=> Use queryset.count() instead of len(queryset.all())
def validate_category_full(category_id):
cat = Category.objects.get(id=category_id)
regs = cat.registration_set.count()
if regs >= cat.max_people:
raise ValidationError('Category is already full.')
Also since you already have the category, you may want to pass it instead of the category.id