# models.py
likes = models.ManyToManyField(settings.AUTH_USER_MODEL, related_name='likes', blank=True)

@property
    def total_likes(self):
        return self.likes.count()

# views
@login_required
@require_POST
def book_like(request):
    if request.method == 'POST':
        user = request.user

        pk = request.POST['pk']
        book = get_object_or_404(Book, pk=pk)

        if book.likes.filter(id=user.id).exists():
            # user has already liked this role game
            # remove like/user
            book.likes.remove(user)
        else:
            # add a new like for a role game
            book.likes.add(user)

        return JsonResponse({'like': role_game.likes.count()})