@transactional Without Save

CartItemsController.kt

@RestController
@RequestMapping("carts/{cart_id}/items")
class CartItemsController(
    private val cartsRepository: CartsRepository,
    private val productsRepository: ProductsRepository) {

    @PostMapping
    fun addItemToCart(
        @PathVariable("cart_id") cartId: UUID,
        @RequestBody cartItem: CartItemInputModel
    ) : ResponseEntity<Any> {
        
        ...

        cart.addProduct(product, cartItem.quantity)
        //cartsRepository.save(cart) // with @Transactional there's no need of this line

        val savedProductItem = cart.items.first { x -> x.product.id == cartItem.productId }
        return ResponseEntity.status(HttpStatus.CREATED)
            .body(mapToCartItemViewModel(savedProductItem))
    }
}