Vouchers losing Django's prefetch_cache on basket loading from cookie

[amatmv]

Issue Summary

In our implementation of oscar ecommerce we have found that, in some cases under heavy load, many baskets are not able to access its vouchers and apply offers.

Steps to Reproduce

We have extended the BasketMiddleware and overridden the method apply_offers_to_basket to add logic to this process.
So our method looks like this:

def apply_offers_to_basket(self, request, basket):
    # Clean up not applicable vouchers before using applicator
    if basket and basket.id:
        for voucher in basket.vouchers.all():
            can_be_applied, __ = voucher.can_be_applied(user=basket.owner)
            if not can_be_applied:
                basket.vouchers.remove(voucher)

We are facing many exceptions in the basket.voucher.all() part, where Django loses the _prefetched_objects_cache and it cannot access its Vouchers.

Possible solution

We believe that we could fix this issue by prefetching the vouchers in the get_basket method in BasketMiddleware.

django-oscar/src/oscar/apps/basket/middleware.py

Lines 132 to 135 in 3dee0ae

	num_baskets_merged = 0
	manager = Basket.open
	cookie_key = self.get_cookie_key(request)
	cookie_basket = self.get_cookie_basket(cookie_key, request, manager)

So the fix could look like

manager = Basket.open.prefetch_related('vouchers')

Technical details

• Python version: 3.9.
• Django version: 3.2
• Oscar version: 2.2.