Django create object only once by a user id

I'm making a little vote system, in wich one can vote an answer, but only once. I have not created a form for the vote, i am creating the vote when someone submits it (accesses a link) How can i make, in my situation, for a p开发者_C百科erson to be able to vote only once? It would be better if i would make a form instead? my code:

def voteup(request,classname, id):
   answer = Answer.objects.get(pk = id)
   c = Classroom.objects.get(classname = classname)
   voted_up_by = Membership.objects.filter(classroom = c).get(member = request.user)
   vote = VoteUpAnswer.objects.create(answer = answer, voted_up_by = voted_up_by) 
   url = reverse('inside_classroom', kwargs={'classname': classname})
   return HttpResponseRedirect(url)

thanks!

I assume that voted_up_by kind of defines the user that voted. If that's true, you can check whether a vote has been added yet. For example

try:
    VoteUpAnswer.objects.get(voted_up_by = voted_up_by)
except VoteUpAnswer.DoesNotExist:
    # No vote from "voted_up_by" exists
    VoteUpAnswer.objects.create(answer = answer, voted_up_by = voted_up_by)
else:
    # User already voted. Redirect to an error page, for example.