问每次单击add to cart按钮时都会出现错误

EN

from cart.models import CartItems
from catalog.models import Product
from django.shortcuts import get_object_or_404
from django.http import HttpResponseRedirect
import decimal
import random

CART_ID_SESSION_KEY = 'cart_id'
# get the current user's cart id, sets new one if blank
def _cart_id(request):
    if request.session.get(CART_ID_SESSION_KEY,'') == '':
        request.session[CART_ID_SESSION_KEY] = _generate_cart_id()
    return request.session[CART_ID_SESSION_KEY]


def _generate_cart_id():
    cart_id = ''
    characters =
'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890!@#$%^&*()'
    cart_id_length = 50
    for y in range(cart_id_length):
        cart_id += characters[random.randint(0, len(characters)-1)]
    return cart_id

# return all items from the current user's cart
def get_cart_items(request):
    return CartItems.objects.filter(cart_id=_cart_id(request))


# add an item to the cart
def add_to_cart(request):
    postdata = request.POST.copy()
    # get product slug from post data, return blank if empty
    product_slug = postdata.get('self.slug','')
    # get quantity added, return 1 if empty
    quantity = postdata.get('quantity',1)
    # fetch the product or return a missing page error
    p = get_object_or_404(Product, slug=product_slug)
    #get products in cart
    cart_products = get_cart_items(request)
    product_in_cart = False
    # check to see if item is already in cart
    for cart_item in cart_products:
        if cart_item.product.id == p.id:
        # update the quantity if found
            cart_item.augment_quantity(quantity)
            product_in_cart = True
    if not product_in_cart:
        # create and save a new cart item
        ci = CartItems()
        ci.product = p
        ci.quantity = quantity
        ci.cart_id = _cart_id(request)
        ci.save()


# returns the total number of items in the user's cart
def cart_distinct_item_count(request):
    return get_cart_items(request).count()

models.py:   
from django.db import models
from catalog.models import Product

class CartItems(models.Model):
    cart_id = models.CharField(max_length=50)
    date_added = models.DateTimeField(auto_now_add=True)
    quantity = models.IntegerField(default=1)
    product = models.ForeignKey(Product, unique=False, 
on_delete=models.CASCADE)

    class Meta:
        db_table = 'cart_items'
        ordering = ['date_added']

    def total(self):
        return self.quantity * self.product.price

    def name(self):
        return self.product.name

    def price(self):
        return self.product.price

    def get_absolute_url(self):
        return self.product.get_absolute_url

   from django.urls import path
   from cart import views

   urlpatterns = [
       path('', views.show_cart, name='show_cart')
    ]

from django.shortcuts import render
from django.template import RequestContext
from cart import cart

def show_cart(request):
    cart_item_count = cart.get_cart_items(request)
    context_instance = RequestContext(request)
    page_title = "showing Cart"
    context = {'page_title' : page_title,
                'cart_item_count': cart_item_count,
                'context_instance': context_instance
                }
    template = 'cart/cart.html'
    return render(request, template, context)

    from django.db import models
    from django.urls import reverse

class Category(models.Model):
    name = models.CharField(max_length=50, db_index=True)
    slug = models.SlugField(max_length=50, unique=True, help_text="Unique 
           value for product page URL, created from name.")
    description = models.TextField(blank=False)
    is_active = models.BooleanField(default=True)
    meta_keywords = models.CharField("Meta Keywords", max_length=255, 
         help_text="Comma-delimited set of SEO keywords for meta tag")
    meta_description = models.CharField("Meta Description", max_length=255, 
                    help_text="Content for description meta tag")
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)

    class Meta:
        verbose_name = "category"
        ordering = ["-created_at"]
        verbose_name_plural = "Categories"

    def __str__(self):
        return self.name

    def get_absolute_url(self):
        return reverse('catalog:show_category', args=[self.slug])


class Product(models.Model):
    name = models.CharField(max_length=255, unique=True)
    slug = models.SlugField(max_length=255, unique=True, help_text="Unique 
           value for product page URL, created from name.")
    brand = models.CharField(max_length=50)
    sku = models.CharField(max_length=50)
    price = models.DecimalField(max_digits=9, decimal_places=2)
    old_price = models.DecimalField(max_digits=9, decimal_places=2, 
               blank=True, default=0.00)
    image = models.ImageField(upload_to='media', blank=True)
    is_active = models.BooleanField(default=True)
    is_bestseller = models.BooleanField(default=False)
    is_featured = models.BooleanField(default=False)
    quantity = models.IntegerField()
    description = models.TextField(blank=False)
    meta_keywords = models.CharField(max_length=255, help_text = 'Comma- 
              delimited set of SEO keywords for meta tag')
    meta_description = models.CharField(max_length=255, help_text = 'Content 
                     for description meta tag')
    created_at = models.DateTimeField(auto_now_add=True)
    updated_at = models.DateTimeField(auto_now=True)
    categories = models.ManyToManyField(Category)

    class Meta:
        db_table = 'products'

        ordering = ['-created_at']

    def __str__(self):
        return self.name

    def get_absolute_url(self):
        return reverse('catalog:show_product', args=[self.slug])

    def sale_price(self):
        if self.old_price > self.price:
            return self.price
        else:
            return None

    from django.shortcuts import render, get_object_or_404, redirect
    from catalog.models import Category, Product
    from django.template import RequestContext
    from django.urls import reverse
    from cart import cart
    from django.http import HttpResponseRedirect
    from catalog.forms import ProductAddToCartForm

def index(request):
    context_instance = RequestContext(request)
    template = "catalog/index.html"
    page_title = 'Musical Instruments and Sheet Music for Musicians'
    context = {'page_title': page_title,
           'context_instance': context_instance}
    return render(request, template, context)


def show_category(request, category_slug):
    template = "catalog/category.html"
    context_instance = RequestContext(request)
    c = get_object_or_404(Category, slug=category_slug)
    products = c.product_set.all()
    page_title = c.name
    meta_keywords = c.meta_keywords
    meta_description = c.meta_description
    context = {
        'context_instance': context_instance,
        'c': c,
        'products': products,
        'page_title': page_title,
        'meta_keywords': meta_keywords,
        'meta_description': meta_description,
    }
    return render(request, template, context)


def show_product(request, product_slug):
    p = get_object_or_404(Product, slug=product_slug)
    context_instance = RequestContext(request)
    categories = p.categories.filter(is_active=True)
    page_title = p.name
    meta_keywords = p.meta_keywords
    meta_description = p.meta_description
    if request.method == 'POST':
        postdata = request.POST.copy()
        form = ProductAddToCartForm(request, postdata)
        if form.is_valid():
            cart.add_to_cart(request)
            if request.session.test_cookie_worked():
                request.session.delete_test_cookies()
            return redirect('cart: show_cart')
    else:
        form = ProductAddToCartForm(request=request, label_suffix=':')
    form.fields['product_slug'].widget.attrs['value'] = product_slug
    request.session.set_test_cookie()
    context = {
        'context_instance': context_instance,
        'p': p,
        'categories': categories,
        'page_title': page_title,
        'meta_keywords': meta_keywords,
        'meta_description': meta_description,
        'form':form
    }
    template = "catalog/product.html"
    return render(request, template, context)

    from django.urls import path
    from catalog import views

app_name = 'catalog'
urlpatterns = [
    path('', views.index, name='index'),
    path('<category_slug>/', views.show_category, name='show_category'),
    path('product/<product_slug>', views.show_product, name='show_product'),

]

    {% extends "catalog.html" %}
    {% block content %}
    {% block sidebar %}
    {% endblock %}
    <div class="container mt-2 pl-5">
<div class="row text-justify ml-5 pl-5">
    <div class="col-md img-responsive">
        <img src="{{ p.image.url }}" alt="{{ p.name }}" class="img-responsive 
center-block img-thumbnail"  width="100%"/>
    </div>
    <div class="col-md">
        <h4>{{ p.name }}</h4>
        Brand: <em>{{ p.brand }}</em>
        <br /><br />
        SKU: {{ p.sku }}
        <br />
        In categor{{ categories.count|pluralize:"y,ies" }}:
            {% for c in categories %}
        <a href="{{ c.get_absolute_url }}">{{ c.name }}</a>
        {% if not forloop.last %}, {% endif %}
            {% endfor %}
            <br /><br />
        {% if p.sale_price %}
        Was: <s>$ {{ p.old_price }}</s>
        <br />
        Now: $ {{ p.price }}
        {% else %}
        Price: $ {{ p.price }}
        {% endif %}
        <br /><br />
<form method="POST" action=".">
    {{ form.as_p }}
     {% csrf_token %}
    <br />
    <input type="submit" value="Add To Cart" name="submit" alt="Add To Cart" 
/>
</form>
    </div>
</div>

<div class=" text-center pt-5 mb-5 pb-5">
    <h3>Product Description</h3>
    {{ p.description }}
</div>
    </div>
 {% endblock %}

Page not found (404)
    Request Method: POST
    Request URL:http://127.0.0.1:8000/catalog/product/
    Raised by:catalog.views.show_category