我正在为民意调查应用程序创建API，但我遇到了这个错误：

  

error.TypeError：“函数”类型的参数不可迭代：django   错误的模型：

我该如何处理？似乎代码中没有错误。

代码：

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


class Poll(models.Model):
    question = models.CharField(max_length=100)
    created_by = models.ForeignKey(User,on_delete=models.CASCADE)
    pub_date = models.DateTimeField(auto_now=True)

def __str__(self):
    return self.question


class Choice(models.Model):
    poll = models.ForeignKey(Poll,related_name='choices',on_delete=models.CASCADE)
    choice_text = models.CharField(max_length=100)

def __str__(self):
    return self.choice_text


class Vote(models.Model):
    choice = models.ForeignKey(Choice,related_name='votes',on_delete=models.CASCADE)
    poll = models.ForeignKey(Poll,on_delete=models.CASCADE)
    voted_by = models.ForeignKey(User,on_delete=models.CASCADE)

class Meta:
    unique_together = ("poll","voted_by")

我的观点：

from django.shortcuts import render,get_object_or_404
    from django.http import JsonResponse

from .models import Poll

def polls_list(request):
    MAX_OBJECTS = 20
    polls = Poll.objects.all()[:MAX_OBJECTS]
    data = {"results": list(polls.values("question","created_by__username","pub_date"))}
    return JsonResponse(data)


def polls_detail(request,pk):
    poll = get_object_or_404(Poll,pk=pk)
    data = {"results": {
        "question": poll.question,"created_by": poll.created_by.username,"pub_date": poll.pub_date
     }}
   return JsonResponse(data)