如果用户不执行任何选择或POST的操作，则Django会检索错误消息

Question

我试图查看用户何时单击“提交”按钮而不进行发布-它将显示错误消息，通知用户单击选项值。它给出了一个错误，指出“地点”处的MultikeyValue错误。当我选择时，它可以进行POST，但是我试图检索错误消息，以防用户未选择任何消息。

models.py

class Ticket(models.Model):
    venue=models.CharField(max_length=100)
    quantity=models.PositiveIntegerField()
    price=models.DecimalField(default=25.00, max_digits=5, decimal_places=2, null=True, blank=True)
    loop=models.CharField(max_length=100)
    purchaser = models.ForeignKey(User, related_name="purchases", on_delete=models.PROTECT)
    created_at=models.DateTimeField(auto_now_add=True)
    updated_at=models.DateTimeField(auto_now=True)

dashboard.html

<form action="/add" method="POST">
                    {% csrf_token %}
                    <div class="text-center">
                            {% if messages %}
                            {% for message in messages %}
                            <div class="alert alert-danger p-2 pb-3">
                                <a class="close font-weight-normal initialism" data-dismiss="alert" href="#"><samp>×</samp></a> 
                                {{message}}
                            </div>
                            {% endfor %}
                        {% endif %}
                        <label><strong>Location of Venue:</strong></label>
                        <select class="form-control" name="venue">
                            <option value="" selected disabled>Please select</option>
                            <option value="San Bernardino">San Bernardino</option>
                            <option value="Los Angeles">Los Angeles</option>
                            <option value="Riverside">Riverside</option>
                        </select>
<button type="submit" value="/add" class="btn btn-dark">Buy Tickets</button>
                </form>

views.py

def add(request):
    if not 'user_id' in request.session:
        return redirect('/chrisgrafil')
    if request.method!='POST':
        return redirect ('/dashboard')
    error=False
    if len(request.POST['venue'])<1:
        messages.error(request, "Please select venue")
        error=True
    if len(request.POST['quantity'])<1:
        messages.error(request, "Please select quantity")
        error=True
    if len(request.POST['loop'])<1:
        messages.error(request, "Please select loop")
        error=True
    else:
        Ticket.objects.create(venue=request.POST['venue'], quantity=request.POST['quantity'], loop=request.POST['loop'], purchaser=User.objects.get(id=request.session['user_id']))
        return redirect ('/confirmation')