parent item mutations

pkg/cart/mutation_items.go

@@ -108,6 +108,7 @@ func (c *CartMutationContext) AddItem(g *CartGrain, m *cart_messages.AddItem) er
 			}
 			existing.ReservationEndTime = endTime
 		}
+		oldQty := existing.Quantity
 		existing.Quantity += uint16(m.Quantity)
 		existing.Stock = uint16(m.Stock)
 		existing.InventoryTracked = m.InventoryTracked
@@ -124,6 +125,20 @@ func (c *CartMutationContext) AddItem(g *CartGrain, m *cart_messages.AddItem) er
 		if len(m.CustomFields) > 0 {
 			existing.CustomFields = m.CustomFields
 		}
+		// Re-add grew the parent's qty — rescale direct child quantities
+		// proportionally so a "+1 drill" keeps its accessories consistent.
+		// Skip when the merged line IS itself a child (ParentId != nil):
+		// cascading up the tree isn't a thing — the parent would not know
+		// a child grew without re-running the promotion engine.
+		if existing.ParentId == nil {
+			c.CascadeChildQuantities(ctx, g, existing.Id, oldQty, existing.Quantity)
+		}
+		// Recompute totals so TotalPrice/TotalDiscount/Discount annotations
+		// reflect the merged parent + cascaded children. The non-merge
+		// (new-line) path below also calls UpdateTotals; the merge path
+		// would otherwise drift after a "+1" re-add, especially now that
+		// cascade may have rewritten N child quantities.
+		g.UpdateTotals()
 
 		return nil
 	}