FieldError at /instagram/post/

안녕하세요, 선생님!

오늘도 신기한 기능 알려주셔서 감사합니다. 

그런데 model.py와 view.py 수정 후 migrate까지 했는데도, FieldError가 발생합니다. create_at에 문제가 있다고 추측만가능하고, 해결방법을 찾지 못했습니다.

정확하게 이 에러가 어떤 상황에서 발생하는지,
어떻게 해결할 수 있는지 조언해주실 수 있으신가요?

에러화면

view.py

from datetime import timedelta
from django.contrib import messages
from django.contrib.auth import get_user_model
from django.contrib.auth.decorators import login_required
from django.db.models import Q
from django.shortcuts import render, redirect, get_object_or_404
from django.utils import timezone
from .forms import PostForm
from .models import Post


@login_required
def index(request):
    timesince = timezone.now() - timedelta(days=3)
    post_list = Post.objects.all()\
        .filter(
            Q(author=request.user) |
            Q(author__in=request.user.following_set.all())
        )\
        .filter(
            created_at__gte=timesince
        )

    suggested_user_list = get_user_model().objects.all()\
        .exclude(pk=request.user.pk)\
        .exclude(pk__in=request.user.following_set.all())[:3]


    return render(request, "instagram/index.html", {
        "post_list": post_list,
        "suggested_user_list": suggested_user_list,
    })

models.py

import re
from django.conf import settings
from django.db import models
from django.urls import reverse


# Create your models here.
class BaseModel(models.Model):
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

    class Meta:
        abstract = True


class Post(models.Model):
    author = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.CASCADE)
    photo = models.ImageField(upload_to = "instagram/post/%Y/%m/%d")
    caption = models.TextField(max_length=500)
    tag_set = models.ManyToManyField('Tag', blank = True)
    location = models.CharField(max_length=100)

    def __str__(self):
        return self.caption