How to use the pretix.base.models.CartPosition.objects.filter function in pretix

def positions(self):
        return CartPosition.objects.filter(
            Q(cart_id=self.cart_id) & Q(event=self.event)
        ).select_related('item', 'subevent')

def _ajax_response_data(self):
        cart_id = get_or_create_cart_id(self.request)
        return {
            'cart_id': cart_id,
            'has_cart': CartPosition.objects.filter(cart_id=cart_id, event=self.request.event).exists()
        }

def count_in_cart(self, now_dt: datetime=None) -> int:
        from pretix.base.models import CartPosition

        now_dt = now_dt or now()
        return CartPosition.objects.filter(
            Q(event=self.event) & Q(subevent=self.subevent) &
            Q(expires__gte=now_dt) &
            Q(
                Q(voucher__isnull=True)
                | Q(voucher__block_quota=False)
                | Q(voucher__valid_until__lt=now_dt)
            ) &
            self._position_lookup
        ).count()

if "cart_id" not in request.GET:
            cached_data = cache.get(cache_key)
            if cached_data:
                return self.response(cached_data)

        data = {
            'currency': request.event.currency,
            'display_net_prices': request.event.settings.display_net_prices,
            'show_variations_expanded': request.event.settings.show_variations_expanded,
            'waiting_list_enabled': request.event.settings.waiting_list_enabled,
            'voucher_explanation_text': str(request.event.settings.voucher_explanation_text),
            'error': None,
            'cart_exists': False
        }

        if 'cart_id' in request.GET and CartPosition.objects.filter(event=request.event, cart_id=request.GET.get('cart_id')).exists():
            data['cart_exists'] = True

        ev = self.subevent or request.event
        data['name'] = str(ev.name)
        fail = False

        if not ev.presale_is_running:
            if ev.presale_has_ended:
                if request.event.settings.presale_has_ended_text:
                    data['error'] = str(request.event.settings.presale_has_ended_text)
                else:
                    data['error'] = ugettext('The presale period for this event is over.')
            elif request.event.settings.presale_start_show_date:
                data['error'] = ugettext('The presale for this event will start on %(date)s at %(time)s.') % {
                    'date': date_format(ev.presale_start.astimezone(request.event.timezone), "SHORT_DATE_FORMAT"),
                    'time': date_format(ev.presale_start.astimezone(request.event.timezone), "TIME_FORMAT"),

def get_cart(request):
    from pretix.presale.views.cart import get_or_create_cart_id

    if not hasattr(request, '_cart_cache'):
        cart_id = get_or_create_cart_id(request, create=False)
        if not cart_id:
            request._cart_cache = CartPosition.objects.none()
        else:
            request._cart_cache = CartPosition.objects.filter(
                cart_id=cart_id, event=request.event
            ).order_by(
                'item', 'variation'
            ).select_related(
                'item', 'variation', 'subevent', 'subevent__event', 'subevent__event__organizer',
                'item__tax_rule'
            )
            for cp in request._cart_cache:
                cp.event = request.event  # Populate field with known value to save queries
    return request._cart_cache

This method migrates legacy sessions created before the upgrade to 1.8.0 on a best-effort basis,
    meaning that the migration does not respect plugin-specific data and works best if the user only
    used the session for one event at the time of migration.

    If ``create`` is ``False`` and no session currently exists, ``None`` will be returned.
    """
    session_keyname = 'current_cart_event_{}'.format(request.event.pk)
    prefix = ''
    if request.resolver_match and request.resolver_match.kwargs.get('cart_namespace'):
        session_keyname += '_' + request.resolver_match.kwargs.get('cart_namespace')
        prefix = request.resolver_match.kwargs.get('cart_namespace')

    current_id = orig_current_id = request.session.get(session_keyname)
    if prefix and 'take_cart_id' in request.GET:
        pos = CartPosition.objects.filter(event=request.event, cart_id=request.GET.get('take_cart_id'))
        if request.method == "POST" or pos.exists():
            current_id = request.GET.get('take_cart_id')

    if current_id and current_id in request.session.get('carts', {}):
        if current_id != orig_current_id:
            request.session[session_keyname] = current_id
        return current_id
    else:
        cart_data = {}
        if prefix and 'take_cart_id' in request.GET and current_id:
            new_id = current_id
            cached_widget_data = widget_data_cache.get('widget_data_{}'.format(current_id))
            if cached_widget_data:
                cart_data['widget_data'] = cached_widget_data
        else:
            if not create:

def _get_voucher_availability(self):
        vouchers_ok = {}
        self._voucher_depend_on_cart = set()
        for voucher, count in self._voucher_use_diff.items():
            voucher.refresh_from_db()

            if voucher.valid_until is not None and voucher.valid_until < self.now_dt:
                raise CartError(error_messages['voucher_expired'])

            redeemed_in_carts = CartPosition.objects.filter(
                Q(voucher=voucher) & Q(event=self.event) &
                Q(expires__gte=self.now_dt)
            ).exclude(pk__in=[
                op.position.id for op in self._operations if isinstance(op, self.ExtendOperation)
            ])
            cart_count = redeemed_in_carts.count()
            v_avail = voucher.max_usages - voucher.redeemed - cart_count
            if cart_count > 0:
                self._voucher_depend_on_cart.add(voucher)
            vouchers_ok[voucher] = v_avail

        return vouchers_ok

def delete(self, request, *args, **kwargs):
        success_url = self.get_success_url()
        o = self.get_object()
        if o.allow_delete():
            CartPosition.objects.filter(addon_to__item=self.get_object()).delete()
            self.get_object().cartposition_set.all().delete()
            self.get_object().log_action('pretix.event.item.deleted', user=self.request.user)
            self.get_object().delete()
            messages.success(request, _('The selected product has been deleted.'))
            return HttpResponseRedirect(success_url)
        else:
            o = self.get_object()
            o.active = False
            o.save()
            o.log_action('pretix.event.item.changed', user=self.request.user, data={
                'active': False
            })
            messages.success(request, _('The selected product has been deactivated.'))
            return HttpResponseRedirect(success_url)