[KrzysztofPajak]

What

A storefront listing validates every applicable discount for every product (DiscountHandlerService.GetAllowedDiscounts). For a discount limited with NTimes or NTimesPerUser, each check sent GetDiscountUsageHistoryQuery with PageSize = 1. That query goes through PagedAsync, so each check cost a sorted find plus a count. A discount assigned to all products therefore cost 48 queries on a page of 24 products.

Changes in DiscountValidationService:

• The usage history is counted with one CountAsync instead of PagedAsync.
• The count is kept for the rest of the scope, keyed by discount and customer. GetProductOverviewHandler prepares product overviews concurrently (Task.WhenAll), so the count Task is stored in a ConcurrentDictionary and concurrent checks share one query.
• ExistsCodeInDiscount uses AnyAsync instead of loading the matching coupons into a list.
• The service takes IRepository<DiscountUsageHistory> instead of IMediator. It already used IRepository<DiscountCoupon> the same way.

The limit semantics are unchanged: the count still includes canceled usages, and NTimesPerUser still filters by the customer.

Nothing is cached between requests. Within one request the count is read once, so a usage written later in that same request is not seen. Order placement validates before it inserts the usage history, so it is not affected.

The DiscountUsageHistory indexes (DiscountId, CustomerId) are already created by the installer, so no migration is needed.

Measured

Local MongoDB, 100,000 usage history documents, one limited discount with 1,000 usages, median of 7 runs, database queries only:

	page of 24 products, before	after
NTimes	47.9 ms (48 queries)	3.3 ms (1 query)
NTimesPerUser	30.8 ms (48 queries)	2.3 ms (1 query)

These are localhost timings. With network latency to the database, the 47 round trips saved are worth more.

Breaking changes

• The DiscountValidationService constructor takes IRepository<DiscountUsageHistory> instead of IMediator. This only affects code that constructs or subclasses the service directly; DI resolves it as before.

Testing

1. dotnet test on Grand.Business.Catalog.Tests (653 passed, 1 skipped) and Grand.Business.Checkout.Tests (292 passed). New tests cover:
   • the NTimes limit, both reached and below
   • NTimesPerUser counting only that customer's usages
   • one count per scope
   • concurrent checks sharing one count
   • ExistsCodeInDiscount
2. PluginDependencyCycleTests (3 passed). The tests needed a mock of the new repository dependency.
3. dotnet build GrandNode.slnx: 0 errors.

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