Cart line changes are not persisted in custom action

Question

I am implementing an action that should add a free item to the cart when the user adds another item.

public class GetFreeItemAction : ICartLineAction
{
    public IRuleValue<string> SellableItemId { get; set; }

    public void Execute(IRuleExecutionContext context)
    {
        var commerceContext = context.Fact<CommerceContext>();

        // this custom method ensures that the ID is in the format CatalogId|ProductId|[VariantId]
        var sellableItemId = SellableItemId.Yield(context).GetCompleteSellableItemId(catalogId);

        var cartLine = new CartLineComponent
        {
            ItemId = sellableItemId,
            Quantity = 1
        };

        cartline.Adjustments?.Add(new CartLineLevelAwardedAdjustment
        {
            Name = "Something",
            DisplayName = "Something else",
            Adjustment = new Money(commerceContext.CurrentCurrency(), cartLine.Totals.SubTotal.Amount * MinusOne),
            AdjustmentType = "Some type",
            IsTaxable = false,
            AwardingBlock = "Some block"
        });

        cartLine.Totals.SubTotal.Amount -= cartLine.Totals.SubTotal.Amount;
        cartLine.GetComponent<MessagesComponent>().AddMessage("Some message code", "Some message");

        cart.Lines.Add(cartLine);
    }
}

If I trigger this action from a promotion that triggers when I am adding an item to the cart, a cart line with Some-SellableItem-Id will also be added to the cart - this cart line is persisted. But the changes I make to the adjustments, totals and messages are not persisted to that cart line component. What do I need to do for this to happen?

UPDATE: I have checked the IAddCartLinePipeline to see if there was something in there, that would remove these changes before they were persisted, but I was not able to find anything of that sort. The closest I came was in CalculateCartLinesTotalsBlock (invoked via IAddCartLinePipeline --> ICalculateCartLinesPipeline) that updates Totals.AdjustmentTotal, Totals.GrandTotal and Totals.PaymentTotal on the cart line - but these updates should not revert my changes.

UPDATE 2: Using Plumber I now have a full overview of the IAddCartLinePipeline contains, including other pipelines that are invoked from this. I cannot find any block that should reset my changes on the promotion-added cart line's adjustment, subtotals and messages. Can anyone spot it?

IAddCartLinePipeline, invokes

• ValidateSellableItemBlock
• AddCartLineBlock
• AddContactBlock
• IPopulateValidateCartPipeline
• AddCartLineGiftCardBlock
• AddCartLineDigitalProductBlock
• AddCartLineWarrantyBlock
• AddCartLineInstallationBlock
• ICalculateCartLinesPipeline, invokes
  • ClearCartLinesBlock
  • ClearCartBlock
  • CalculateCartLinesSubTotalsBlock
  • CalculateCartLinesFulfillmentBlock
  • CalculateCartLinesPromotionsBlock, invokes
    • IEvaluatePromotionsQualificationsPipeline
    • IApplyPromotionsBenefitsPipeline, which somewhere down the line invokes RunRuleSetBlock to apply my promotion, which in turn adds the cart line
  • CalculateCartLinesTaxBlock
  • CalculateCartLinesTotalsBlock
• ICalculateCartPipeline, invokes
  • CalculateCartSubTotalsBlock
  • CalculateCartFulfillmentBlock
  • CalculateCartPromotionsBlock
  • CalculateCartTaxBlock
  • CalculateCartTotalsBlock
  • CalculateCartPaymentsBlock
  • WriteCartTotalsToContextBlock
• PersistCartBlock

Answer 1

Yes the Cart.Adjustments and Cart.Totals objects are calculated, so you should not update them directly.

Instead, you can simply add another promotion benefit to your promotion: "Get Cart Subtotal [specific] Amount Off", and set that to the total of your free item?

That will apply at the same time as your GetFreeItem action (which will just add the new cart item), because both are on the same promotion.