Changing in the Quantity of variants reflecting in the wrong item in Order Summary

I have a problem with the variations and the quantity related to it in the order summary page.

This is an example to simplify the error

when I add to the cart 2 items with different variations:

• Item X with a size small
• Item X with a size medium

When I change the quantity of item X size medium, this change is reflecting in item X size small which was chosen first.

In the order summary, there are a plus and minus in the template to change the quantity.

I have identified the problem but I can't figure out why it is occurring

Here is the template:

        <h2> Order Summary</h2>
            <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> 

Here is the views.py

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)
# End Remove Items (Products removed from Cart)

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}"