LazyInitializationException

CartItemsControllerTests.kt

val product =  productsRepository.findByIdOrNull(cartItem.productId)
if (product == null) {
    return ResponseEntity.notFound().build()
}

val cart = cartsRepository.findByIdOrNull(cartId)
if (cart == null) {
    return ResponseEntity.notFound().build()
}

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