如何限制匿名用户在django投票应用中只能投票一次？

# models.py
from django.db import models
from django.contrib.auth.models import User

class Poll(models.Model):
    question = models.CharField(max_length=200)
    pub_date = models.DateTimeField('date published')
    voters = models.ManyToManyField(User)

class Choice(models.Model):
    poll = models.ForeignKey(Poll, on_delete=models.CASCADE)
    choice_text = models.CharField(max_length=200)
    votes = models.IntegerField(default=0)

# views.py
from django.shortcuts import render, get_object_or_404, redirect
from django.contrib.auth.decorators import login_required

@login_required
def vote(request, poll_id):
    poll = get_object_or_404(Poll, pk=poll_id)
    if request.user in poll.voters.all():
        return render(request, 'polls/error.html', {'error_message': "You have already voted."})
    else:
        selected_choice = poll.choice_set.get(pk=request.POST['choice'])
        selected_choice.votes += 1
        selected_choice.save()
        poll.voters.add(request.user)
        return redirect('polls:results', poll_id=poll.id)