非空约束失败:门户\访问请求.访问者\ id

我是django的新手,我已经为这个项目奋斗了一段时间。每次尝试提交表单(最终创建访问者和访问者请求对象)时,我都会收到一个NOTNULL约束错误。错误来自这行代码。。。 访问请求=访问请求(staff=staff,visitor=visitor,comment=comment,token=token,status=None)。保存() ... 请查看下面的代码。

views.py

def formpage(request):
    if request.method=='POST':
        token=secrets.token_urlsafe(20)
        visitor_name=request.POST.get('visitorsname')
        comment=request.POST.get('comment')
        visit_type=request.POST.get('visit_type')
        visit_content='You have a waiting visitor'+'\n'+'Name:'+visitor_name+'\n'+'Purpose Of Visit:'+visit_type+'\n'+'Additional Comment:'+comment+'\n'+token
        staff_id=request.POST.get('staff')
        staff=Staff.objects.get(id=staff_id)
        staff_email=staff.staff_email
        req_comment = request.POST.get('req_comment')
        request_id = (request.POST.get('request_id'))
        visitor=Visitor(visitor_name=visitor_name).save()
        visit_request = VisitRequests(staff=staff, visitor=visitor, comment=comment, token=token, status=None).save()

models.py

class Staff(models.Model):
    staff_name = models.CharField(max_length=250)
    staff_email = models.CharField(max_length=250, default="")

    def __str__(self):
        return self.staff_name

class Visitor(models.Model):
    visitor_name = models.CharField(max_length=250)
    timestamp = models.DateTimeField(default=timezone.now)

    def __str__(self):
        return '{}'.format(self.visitor_name)

class VisitRequests(models.Model):
    staff=models.ForeignKey(Staff, on_delete=models.CASCADE)
    visitor = models.ForeignKey(Visitor, on_delete=models.CASCADE)
    comment= models.TextField(default='')
    status= models.NullBooleanField()
    token=models.CharField(max_length=20)