functions.fsl

function createOrUpdateCartItem(customerId, productName, quantity) {
  // Find the customer by id, using the ! operator to assert that the customer exists.
  // If the customer does not exist, fauna will throw a document_not_found error.
  let customer = Customer.byId(customerId)!
  // There is a unique constraint on [.name] so this will return at most one result.
  let product = Product.byName(productName).first()

  // Check if the product exists.
  if (product == null) {
    abort("Product does not exist.")
  }

  // Check that the quantity is valid.
  if (quantity < 0) {
    abort("Quantity must be a non-negative integer.")
  }

  // Create a new cart for the customer if they do not have one.
  if (customer!.cart == null) {
    Order.create({
      status: "cart",
      customer: customer,
      createdAt: Time.now(),
      payment: {}
    })
  }

  // Check that the product has the requested quantity in stock.
  if (product!.stock < quantity) {
    abort("Product does not have the requested quantity in stock.")
  }

  // Attempt to find an existing order item for the order, product pair.
  // There is a unique constraint on [.order, .product] so this will return at most one result.
  let orderItem = OrderItem.byOrderAndProduct(customer!.cart, product).first()

  if (orderItem == null) {
    // If the order item does not exist, create a new one.
    OrderItem.create({
      order: Order(customer!.cart!.id),
      product: product,
      quantity: quantity,
    })
  } else {
    // If the order item exists, update the quantity.
    orderItem!.update({ quantity: quantity })
  }
  // Return the customer's updated cart.
  customer!.cart
}

function getOrCreateCart(id) {
  // Find the customer by id, using the ! operator to assert that the customer exists.
  // If the customer does not exist, fauna will throw a document_not_found error.
  let customer = Customer.byId(id)!

  if (customer!.cart == null) {
    // Create a cart if the customer does not have one.
    Order.create({
      status: 'cart',
      customer: Customer.byId(id),
      createdAt: Time.now(),
      payment: {}
    })
  } else {
    // Return the cart if it already exists.
    customer!.cart
  }
}

function checkout(orderId, status, payment) {
  // Find the order by id, using the ! operator to assert that the order exists.
  let order: Any = Order.byId(orderId)!

  // Check that we are setting the order to the processing status. If not, we should
  // not be calling this function.
  if (status != "processing") {
    abort("Can not call checkout with status other than processing.")
  }

  // Check that the order can be transitioned to the processing status.
  validateOrderStatusTransition(order!.status, "processing")

  // Check that the order has at least one order item.
  if (order!.items.isEmpty()) {
    abort("Order must have at least one item.")
  }

  // Check that customer has a valid address.
  if (order!.customer!.address == null) {
    abort("Customer must have a valid address.")
  }

  // Check that the order has a payment method if not provided as an argument.
  if (order!.payment == null && payment == null) {
    abort("Order must have a valid payment method.")
  }

  // Check that the order items are still in stock.
  order!.items.forEach((item) => {
    let product: Any = item.product
    if (product.stock < item.quantity) {
      abort("One of the selected products does not have the requested quantity in stock.")
    }
  })

  // Decrement the stock of each product in the order.
  order!.items.forEach((item) => {
    let product: Any = item.product
    product.update({ stock: product.stock - item.quantity })
  })

  // Transition the order to the processing status, update the payment if provided.
  if (payment != null) {
    order!.update({ status: "processing", payment: payment })
  } else {
    order!.update({ status: "processing" })
  }
}

function validateOrderStatusTransition(oldStatus, newStatus) {
  if (oldStatus == "cart" && newStatus != "processing") {
    // The order can only transition from cart to processing.
    abort("Invalid status transition.")
  } else if (oldStatus == "processing" && newStatus != "shipped") {
    // The order can only transition from processing to shipped.
    abort("Invalid status transition.")
  } else if (oldStatus == "shipped" && newStatus != "delivered") {
    // The order can only transition from shipped to delivered.
    abort("Invalid status transition.")
  }
}