Rails Service Object

Rails Service Object Pattern

Overview

Service objects encapsulate business logic:

• Single responsibility (one public method: #call)
• Easy to test in isolation
• Reusable across controllers, jobs, rake tasks
• Clear input/output contract
• Dependency injection for testability

When to Use Service Objects

When NOT to Use Service Objects

Don't create a service object when:

• A model callback does the job (e.g., after_create :send_welcome_email)
• The logic is a single ActiveRecord operation
• A concern would share the behavior more naturally
• You're wrapping a single method call (adds indirection for no benefit)
• The "service" just delegates to one model method

Rule of thumb: If your service object's #call method is under 5 lines and calls one model method, you don't need it.

Workflow Checklist

Service Object Progress:
- [ ] Step 1: Define input/output contract
- [ ] Step 2: Create service test (RED)
- [ ] Step 3: Run test (fails - no service)
- [ ] Step 4: Create service file with empty #call
- [ ] Step 5: Run test (fails - wrong return)
- [ ] Step 6: Implement #call method
- [ ] Step 7: Run test (GREEN)
- [ ] Step 8: Add error case tests
- [ ] Step 9: Implement error handling
- [ ] Step 10: Final test run

Step 1: Define Contract

## Service: Orders::CreateService

### Purpose
Creates a new order with inventory validation and payment processing.

### Input
- user: User (required)
- items: Array<Hash> (required) - [{product_id:, quantity:}]
- payment_method_id: Integer (optional)

### Output (Result object)
Success: { success?: true, data: Order }
Failure: { success?: false, error: String, code: Symbol }

### Dependencies
- inventory_service: Checks product availability
- payment_gateway: Processes payment

### Side Effects
- Creates Order and OrderItem records
- Decrements inventory
- Charges payment method
- Sends confirmation email (async)

Step 2: Service Test

Location: test/services/orders/create_service_test.rb

# frozen_string_literal: true

require "test_helper"

class Orders::CreateServiceTest < ActiveSupport::TestCase
  setup do
    @user = users(:one)
    @product = products(:available)
    @items = [{ product_id: @product.id, quantity: 2 }]
    @service = Orders::CreateService.new
  end

  test "#call with valid inputs returns success" do
    result = @service.call(user: @user, items: @items)

    assert result.success?
    assert_instance_of Order, result.data
    assert_equal @user, result.data.user
  end

  test "#call with valid inputs creates an order" do
    assert_difference("Order.count", 1) do
      @service.call(user: @user, items: @items)
    end
  end

  test "#call with empty items returns failure" do
    result = @service.call(user: @user, items: [])

    assert result.failure?
    assert_equal "No items provided", result.error
  end

  test "#call with insufficient inventory returns failure" do
    items = [{ product_id: @product.id, quantity: 999_999 }]

    result = @service.call(user: @user, items: items)

    assert result.failure?
  end

  test "#call with insufficient inventory does not create order" do
    items = [{ product_id: @product.id, quantity: 999_999 }]

    assert_no_difference("Order.count") do
      @service.call(user: @user, items: items)
    end
  end
end

Step 3-6: Implement Service

Location: app/services/orders/create_service.rb

# frozen_string_literal: true

module Orders
  class CreateService
    def initialize(inventory_service: InventoryService.new,
                   payment_gateway: PaymentGateway.new)
      @inventory_service = inventory_service
      @payment_gateway = payment_gateway
    end

    def call(user:, items:, payment_method_id: nil)
      return failure("No items provided", :empty_items) if items.empty?
      return failure("Insufficient inventory", :insufficient_inventory) unless inventory_available?(items)

      order = create_order(user, items)
      process_payment(order, payment_method_id) if payment_method_id

      success(order)
    rescue ActiveRecord::RecordInvalid => e
      failure(e.message, :validation_failed)
    end

    private

    attr_reader :inventory_service, :payment_gateway

    def inventory_available?(items)
      items.all? do |item|
        inventory_service.available?(item[:product_id], item[:quantity])
      end
    end

    def create_order(user, items)
      ActiveRecord::Base.transaction do
        order = Order.create!(user: user, status: :pending)

        items.each do |item|
          order.order_items.create!(
            product_id: item[:product_id],
            quantity: item[:quantity]
          )
          inventory_service.decrement(item[:product_id], item[:quantity])
        end

        order
      end
    end

    def process_payment(order, payment_method_id)
      payment_gateway.charge(
        amount: order.total,
        payment_method_id: payment_method_id
      )
      order.update!(status: :paid)
    end

    def success(data)
      Result.new(success: true, data: data)
    end

    def failure(error, code = :unknown)
      Result.new(success: false, error: error, code: code)
    end
  end
end

Result Object

# app/services/result.rb
# frozen_string_literal: true

class Result
  attr_reader :data, :error, :code

  def initialize(success:, data: nil, error: nil, code: nil)
    @success = success
    @data = data
    @error = error
    @code = code
  end

  def success?
    @success
  end

  def failure?
    !@success
  end

  def deconstruct_keys(keys)
    { success: @success, data: @data, error: @error, code: @code }
  end
end

Testing with Mocked Dependencies

class Orders::CreateServiceTest < ActiveSupport::TestCase
  setup do
    @inventory_service = Minitest::Mock.new
    @payment_gateway = Minitest::Mock.new
    @service = Orders::CreateService.new(
      inventory_service: @inventory_service,
      payment_gateway: @payment_gateway
    )
  end

  test "calls inventory service to check availability" do
    @inventory_service.expect(:available?, true, [Integer, Integer])
    @inventory_service.expect(:decrement, true, [Integer, Integer])

    @service.call(user: users(:one), items: [{ product_id: 1, quantity: 2 }])

    @inventory_service.verify
  end
end

Calling Services

From Controllers

class OrdersController < ApplicationController
  def create
    result = Orders::CreateService.new.call(
      user: current_user,
      items: order_params[:items],
      payment_method_id: order_params[:payment_method_id]
    )

    if result.success?
      redirect_to result.data, notice: "Order created"
    else
      flash.now[:alert] = result.error
      render :new, status: :unprocessable_entity
    end
  end
end

From Jobs

class ProcessOrderJob < ApplicationJob
  def perform(user_id, items)
    user = User.find(user_id)
    result = Orders::CreateService.new.call(user: user, items: items)

    unless result.success?
      Rails.logger.error("Order failed: #{result.error}")
    end
  end
end

Directory Structure

app/services/
  result.rb
  orders/
    create_service.rb
    cancel_service.rb
  users/
    register_service.rb
  payments/
    charge_service.rb

Conventions

1. Naming: Namespace::VerbNounService (e.g., Orders::CreateService)
2. Location: app/services/[namespace]/[name]_service.rb
3. Interface: Single public method #call
4. Return: Always return Result object
5. Dependencies: Inject via constructor
6. Errors: Catch and wrap in Result, don't raise

Anti-Patterns to Avoid

1. God service: Too many responsibilities - split it
2. Hidden dependencies: Using globals instead of injection
3. No return contract: Returning different types
4. Raising exceptions: Use Result objects instead
5. Service wrapping one method: Just call the method directly
6. Service with multiple public methods: Use separate services

Checklist

• Contract defined (input/output/side effects)
• Test written first (RED)
• Single public method #call
• Returns Result object consistently
• Dependencies injected via constructor
• Error cases tested
• Transaction wraps multi-model operations
• All tests GREEN

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