Updating Quantity in an Order Summary

I'm trying to fix a bug in my project.

When a user goes to the product detail page, the user can select a variant size (Small, medium or Large).

Then next goes to the Order Summary page with all the items looped with its variation. If Item 1 with Small and Item 1 with Medium and the quantity for each.

In the Order Summary, there is an option to add or remove single items for each, and this is where the bug is.

Example (the sequence is important):

 - Added Item 1 to cart with a size Small.
 - Added another Item 1 to cart with size Medium

So now in the Order Summary, I have a table with the following

 - Item 1    Size: Small     Quantity: 1
 - Item 1    Size: Medium    Quantity: 1

Now if I want to change the quantity of  Item 1    Size: Medium, the very first item select is the one which is updated:

 - Item 1    Size: Small     Quantity: 2
 - Item 1    Size: Medium    Quantity: 1

The reason behind this is because when add to cart or remove to cart in the template is reflecting the slug of the item not identifying the variation of the selected item and I have tried many times to fix it but still it doesn't reflect correctly.

I have tried to change from slug to id in order to pinpoint the item that needed quantity updating but it didn't work.Here is the Views:
 

class OrderSummaryView(LoginRequiredMixin, View):
    def get(self, *args, **kwargs):

        try:
            order = Order.objects.get(user=self.request.user, ordered=False)
            context = {
                'object': order
            }
            return render(self.request, 'order_summary.html', context)
        except ObjectDoesNotExist:
            messages.warning(self.request, "You do not have an active order")
            return redirect("/")

@login_required
def add_to_cart(request, slug):
    item = get_object_or_404(Item, slug=slug)
    order_item_qs = OrderItem.objects.filter(
        item=item,
        user=request.user,
        ordered=False
    )
    item_var = []  # item variation
    if request.method == 'POST':
        for items in request.POST:
            key = items
            val = request.POST[key]
            try:
                v = Variation.objects.get(
                    item=item,
                    category__iexact=key,
                    title__iexact=val
                )
                item_var.append(v)
            except:
                pass

        if len(item_var) > 0:
            for items in item_var:
                order_item_qs = order_item_qs.filter(
                    variation__exact=items,
                )

    if order_item_qs.exists():
        order_item = order_item_qs.first()
        order_item.quantity += 1
        order_item.save()
    else:
        order_item = OrderItem.objects.create(
            item=item,
            user=request.user,
            ordered=False
        )
        order_item.variation.add(*item_var)
        order_item.save()

    order_qs = Order.objects.filter(user=request.user, ordered=False)
    if order_qs.exists():
        order = order_qs[0]
        # check if the order item is in the order
        if not order.items.filter(item__id=order_item.id).exists():
            order.items.add(order_item)
            messages.info(request, "This item quantity was updated.")
            return redirect("core:order-summary")
    else:
        ordered_date = timezone.now()
        order = Order.objects.create(
            user=request.user, ordered_date=ordered_date)
        order.items.add(order_item)
        messages.info(request, "This item was added to cart.")
        return redirect("core:order-summary")


@login_required
def remove_from_cart(request, slug):
    item = get_object_or_404(Item, slug=slug)
    order_qs = Order.objects.filter(
        user=request.user,
        ordered=False
    )
    if order_qs.exists():
        order = order_qs[0]
        # check if the order item is in the order
        if order.items.filter(item__slug=item.slug).exists():
            order_item = OrderItem.objects.filter(
                item=item,
                user=request.user,
                ordered=False
            )[0]
            order.items.remove(order_item)
            order_item.delete()
            messages.info(request, "This item was removed from your cart")
            return redirect("core:order-summary")

        else:
            messages.info(request, "This item was not in your cart")
            return redirect("core:product", slug=slug)
    else:
        messages.info(request, "You don't have an active order")
        return redirect("core:product", slug=slug)


@login_required
def remove_single_item_from_cart(request, slug):
    item = get_object_or_404(Item, slug=slug)
    order_qs = Order.objects.filter(
        user=request.user,
        ordered=False
    )
    if order_qs.exists():
        order = order_qs[0]
        # check if the order item is in the order
        if order.items.filter(item__slug=item.slug).exists():
            order_item = OrderItem.objects.filter(
                item=item,
                user=request.user,
                ordered=False
            )[0]
            if order_item.quantity > 1:
                order_item.quantity -= 1
                order_item.save()
            else:
                order.items.remove(order_item)
            messages.info(request, "This item quantity was updated")
            return redirect("core:order-summary")
        else:
            messages.info(request, "This item was not in your cart")
            return redirect("core:product", slug=slug)
    else:
        messages.info(request, "You do not have an active order")
        return redirect("core:product", slug=slug)

Here is the template:

    <main>
        <div class="container">
        <div class="table-responsive text-nowrap" style="margin-top:90px">
        <h2> Order Summary</h2>
        <table class="table">
            <thead>
            <tr>
                <th scope="col">#</th>
                <th scope="col">Item Title</th>
                <th scope="col">Price</th>
                <th scope="col">Quantity</th>
                <th scope="col">Size</th> 
                <th scope="col">Total Item Price</th>
            </tr>
            </thead>
            <tbody>
            {% for order_item in object.items.all %}
            <tr>
                <th scope="row">{{ forloop.counter }}</th>
                <td>{{ order_item.item.title }}</td>
                <td>{{ order_item.item.price }}</td>
                <td>
                <a href="{% url 'core:remove-single-item-from-cart' order_item.item.slug %}"><i class="fas fa-minus mr-2"></a></i>
                {{ order_item.quantity }}
                <a href="{% url 'core:add-to-cart' order_item.item.slug %}"><i class="fas fa-plus ml-2"></a></i>
                </td>                
                <td>
                {% if order_item.variation.all %}
                {% for variation in order_item.variation.all %}
                {{ variation.title|capfirst }}
                {% endfor %}
                {% endif %}
                </td> 
                <td>
                {% if order_item.item.discount_price %}
                    $ {{ order_item.get_total_discount_item_price }}
                    <span class="badge badge-primary" style="margin-left:10px">Saving ${{ order_item.get_amount_saved }}</span>
                {% else %}
                    $ {{ order_item.get_total_item_price }}
                {% endif %}
                <a style="color:red" href="{% url 'core:remove-from-cart' order_item.item.slug %}">
                <i class="fas fa-trash float-right"></i>
                </a>
                </td>
            </tr>
            {% empty %}
            <tr>
                <td colspan='5'>Your Cart is Empty</td>
            </tr>
            <tr>
                <td colspan="5">
                <a class='btn btn-primary float-right ml-2'href='/'>Continue Shopping</a>
            </tr>                
            {% endfor %}
            {% if object.coupon %}
            <tr>
                <td colspan="4"><b>Coupon</b></td>
                <td><b>-${{ object.coupon.amount }}</b></td>
            </tr>            
            {% endif %}
            <tr>
                <td colspan="5"><b>Sub total</b></td>
                <td><b>${{ object.get_total }}</b></td>
            </tr>
            <tr>
                <td colspan="5">Taxes</td>
                <td>${{ object.get_taxes|floatformat:2  }}</td>
            </tr>
            {% if object.grand_total %}
            <tr>
                <td colspan="5"><b>Grand Total</b></td>
                <td><b>${{ object.grand_total|floatformat:2 }}</b></td>
            </tr>
            <tr>
                <td colspan="6">
                <a class='btn btn-primary float-right ml-2'href='/'>Continue Shopping</a>
                <a class='btn btn-warning float-right'href='/checkout/'>Proceed to Checkout</a></td>
            </tr> 
            {% endif %}                          
            </tbody>
        </table>
        </div>
        </div>
    </main>

here is the models.py

class Item(models.Model):
    title             = models.CharField(max_length=100)
    -------------------------------------------------------------------------
    updated           = models.DateTimeField(auto_now_add=False, auto_now=True)
    active            = models.BooleanField(default=True)

    def __str__(self):
        return self.title

    def get_absolute_url(self):
        return reverse("core:product", kwargs={
            'slug': self.slug
        })

    def get_add_to_cart_url(self):
        return reverse("core:add-to-cart", kwargs={
            'slug': self.slug
        })

    def get_remove_from_cart_url(self):
        return reverse("core:remove-from-cart", kwargs={
            'slug': self.slug
        })


class VariationManager(models.Manager):
    def all(self):
        return super(VariationManager, self).filter(active=True)

    def sizes(self):
        return self.all().filter(category='size')

    def colors(self):
        return self.all().filter(category='color')


VAR_CATEGORIES = (
    ('size', 'size',),
    ('color', 'color',),
    ('package', 'package'),
)


class Variation(models.Model):
    item = models.ForeignKey(Item, on_delete=models.CASCADE)
    category = models.CharField(
        max_length=120, choices=VAR_CATEGORIES, default='size')
    title = models.CharField(max_length=120)
    image = models.ImageField(null=True, blank=True)
    price = models.DecimalField(
        decimal_places=2, max_digits=100, null=True, blank=True)
    objects = VariationManager()
    active = models.BooleanField(default=True)

    def __str__(self):
        return self.title

class OrderItem(models.Model):
    user = models.ForeignKey(settings.AUTH_USER_MODEL,
                             on_delete=models.CASCADE)
    ordered = models.BooleanField(default=False)
    item = models.ForeignKey(Item, on_delete=models.CASCADE)
    quantity = models.IntegerField(default=1)
    variation = models.ManyToManyField(Variation)

    def __str__(self):
        return f"{self.quantity} of {self.item.title}"

Urls.py

app_name = 'core'

urlpatterns = [
    path('', HomeView.as_view(), name='home'),
    path('order-summary/', OrderSummaryView.as_view(), name='order-summary'),
    path('product/<slug>/', ItemDetailView.as_view(), name='product'),
    path('add-to-cart/<slug>/', add_to_cart, name='add-to-cart'),
    path('remove-item-from-cart/<slug>/', remove_single_item_from_cart,
         name='remove-single-item-from-cart'),
    path('remove-from-cart/<slug>/', remove_from_cart, name='remove-from-cart'),
]