Разобраться с ForeignKey

class ServiceModel(ListView):
    model = Portfolio
    context_object_name = 'gallery_category'
    template_name = 'service/gallery_category.html'
 
    def get_queryset(self):
        return Portfolio.objects.filter(slug=self.kwargs.get('slug'))

class ServiceModel(DetailView):
    model = Portfolio
    template_name = 'service/gallery_category.html'
    slug_url_kwarg = 'slug'

from django.db import models
from django.urls import reverse
 
 
class Portfolio(models.Model):
    title = models.CharField('Название галереи', max_length=100)
    sub_title = models.CharField('Краткое описание', max_length=150)
    images = models.ImageField('Заставка галереи', upload_to='images/%Y/%m/%d')
    slug = models.SlugField('URL', max_length=100, default = '', unique=True)
    description = models.CharField('Description', max_length = 120, blank=True, null=True)
    keywords = models.CharField('Keywords', max_length = 120, blank=True, null=True)
 
    def __str__(self):
        return self.title
    
    @property
    def images_url(self):
        if self.images and hasattr(self.images, 'url'):
            return self.images.url
    
    def get_absolute_url(self):
        return reverse("gallery_category", kwargs={"slug": self.slug})
 
class Gallery(models.Model):
    photos = models.ImageField(upload_to='gallery')
    product = models.ForeignKey(Portfolio, on_delete=models.CASCADE, related_name='photos')

from django.contrib import admin
from gallery.models import Portfolio, Gallery
 
 
class GalleryInline(admin.TabularInline):
    fk_name = 'product'
    model = Gallery
 
 
@admin.register(Portfolio)
class PortfoliotAdmin(admin.ModelAdmin):
    inlines = [GalleryInline,]

from django.shortcuts import render
from .models import Portfolio 
from django.views.generic import DetailView
 
def index(request):
    '''Главная страница'''
    context = {}
    services = Portfolio.objects.all()
    context['services'] = services
    template = 'base.html'
    return render(request, template, context)
 
 
class ServiceModel(DetailView):
    model = Portfolio
    template_name = 'service/gallery_category.html'
    slug_url_kwarg = 'slug'

from django.urls import path
from gallery import views
from .views import ServiceModel
 
 
 
urlpatterns = [
    path('', views.index),
    path('<slug:slug>/', ServiceModel.as_view(), name = 'gallery_category'),
    
]