ruby 55 lines · 9 steps

How a Rails checkout service object works

A plain Ruby service object wraps checkout in a transaction and returns a structured result instead of raising.

Explained by highlit

Rails Intermediate service object transactions result object error handling encapsulation

1class CheckoutService
Result = Struct.new(:success?, :order, :error, keyword_init: true)
3 
def self.call(...)
  new(...).call
end
7 
def initialize(cart:, payment_token:, current_user:)
  @cart = cart
  @payment_token = payment_token
  @current_user = current_user
end
13 
def call
  return failure("Your cart is empty") if @cart.empty?
16 
  order = nil
  ActiveRecord::Base.transaction do
    order = build_order
    order.save!
    reserve_inventory!(order)
    charge = PaymentGateway.charge(token: @payment_token, amount_cents: order.total_cents)
    order.update!(payment_reference: charge.id, status: :paid)
  end
25 
  OrderMailer.confirmation(order).deliver_later
  @cart.clear!
  Result.new(success?: true, order: order)
rescue PaymentGateway::Error => e
  failure("Payment failed: #{e.message}")
rescue InventoryReservation::OutOfStock => e
  failure("#{e.item_name} is out of stock")
end
34 
private
36 
def build_order
  @current_user.orders.build(
    total_cents: @cart.total_cents,
    currency: @cart.currency,
    status: :pending,
    line_items_attributes: @cart.to_line_item_attributes
  )
end
45 
def reserve_inventory!(order)
  order.line_items.each do |item|
    InventoryReservation.reserve!(item.variant, quantity: item.quantity)
  end
end
51 
def failure(message)
  Result.new(success?: false, error: message)
end
55end

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Service objects keep multi-step business logic out of controllers and models in one testable place.
2Wrapping side effects in a transaction guarantees the order, inventory, and payment commit together or not at all.
3Returning a Result struct lets callers branch on success without rescuing exceptions themselves.

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