**What**
I have created a new method on the cart service which is `addLineItems`, allowing a user to add one or multiple items in an optimized way. Also updated the `generate` method from the line item service which now also accept a object data or a collection of data which. Various places have been optimized and cache support has been added to the price selection strategy.
The overall optimization allows to reach another 9000% improvement in the response time as a median (Creating a cart with 6 items):
| | Min (ms) | Median (ms) | Max (ms) | Median Improvement (%)
|---|:-:|---|---|---|
| Before optimisation | 1200 | 9999 | 12698 | N/A
| After optimisation | 63 | 252 | 500 | 39x
| After re optimisation | 56 | 82 | 399 | 121x
| After including addressed feedback | 65 | 202 | 495 | 49x
FIXES CORE-722
**What**
Fix system error (500) with DraftOrder create operation when payload includes discount in a tax-inclusive context.
**How**
* Ensure newly created cart contains all required relation in order to calculate line item tax-inclusive pricing with discounts.
* Add resilience to TotalsService.getLineDiscounts()
* Ensure newly generate line items have variant relation loaded.
* fix TotalsService.getLineItemTotals to use the passed lineItem instead of relying on cartOrOrder.items.
**Test**
* Unit:
* TotalsService.getLineDiscounts - coverage
* Integration:
* Admin API draft-order - coverage
* Admin API draft-order create w/ discount in tax-inclusive
Resolves: CORE-771
Co-authored-by: Oliver Windall Juhl <59018053+olivermrbl@users.noreply.github.com>
Co-authored-by: Adrien de Peretti <25098370+adrien2p@users.noreply.github.com>
