Djangoで外部キーを設定してCRUDしたい

前提・実現したいこと

Djangoで外部キーを設定してCRUDしたいのですが追加及び更新にてエラーが出力されます。

発生している問題・エラーメッセージ

ValueError at /securitycard/add/
Cannot assign "<django.db.models.query_utils.DeferredAttribute object at 0x1054da320>": "SecurityCardUser.securitycard" must be a "SecurityCard" instance.

該当のソースコード

python
1from django.http import HttpResponse, HttpResponseRedirect
2from django.shortcuts import render
3from django.utils import timezone
4from securitycard.models import SecurityCard, SecurityCardUser, SecurityCardForm
5from django.urls import reverse
6
7# Create your views here.
8def index(request):
9    security_list = SecurityCard.objects.all()
10    context = {'security_list': security_list}
11    return render(request, 'securitycard/index.html', context)
12
13def new(request):
14    return render(request, 'securitycard/new.html')
15
16def add(request):
17    t1 = SecurityCardUser(securitycard=SecurityCard.card_number)
18#    t1 = SecurityCardUser()
19#    t1.card_number = len(SecurityCard.objects.order_by('-card_number'))+1
20#    t1.update_date = timezone.now()
21    t = SecurityCardForm(request.POST, instance=t1)
22    t.save()
23    return HttpResponseRedirect(reverse('index'))
24
25def detail(request, securitycard_id):
26    securitycard = SecurityCard.objects.get(card_number=securitycard_id)
27    context = {'securitycard': securitycard}
28    return render(request, 'securitycard/new.html', context)
29
30def update(request,securitycard_id):
31    t = SecurityCardUser()
32    t1 = SecurityCardForm(request.POST, instance=t)
33    if t1.is_valid():
34        t=t1.save(commit=False)
35        t.save()
36        return HttpResponseRedirect(reverse('index'))
37
38def delete(request, securitycard_id):
39    t = SecurityCard.objects.get(card_number=securitycard_id)
40    t.delete()
41    return HttpResponseRedirect(reverse('index'))

python
1from django.db import models
2from django.forms import ModelForm
3
4# Create your models here.
5class SecurityCard(models.Model):
6    card_number= models.IntegerField(primary_key = True)
7
8class SecurityCardUser(models.Model):
9    securitycard = models.ForeignKey(SecurityCard, on_delete = models.CASCADE)
10    user_name = models.CharField(max_length = 50)
11    checkout_at = models.CharField(max_length=50)
12    return_at = models.CharField(max_length=50)
13
14#class SecurityCardForm(ModelForm):
15#    class Meta:
16#        model = SecurityCard 
17#        fields = ['card_number']
18
19class SecurityCardForm(ModelForm):
20    class Meta:
21        model = SecurityCardUser 
22        fields = ['securitycard','user_name','checkout_at','return_at']