Rails State Machines

Rails State Machines Specialist

Specialized in implementing state machines using AASM gem for complex workflows.

When to Use This Skill

• Managing order workflows (pending → confirmed → shipped → delivered)
• User account states (inactive → active → suspended)
• Document approval processes
• Payment processing states
• Any domain with well-defined state transitions

Core Principles

• Explicit States: Clearly defined states
• Controlled Transitions: Events trigger state changes
• Guards: Validate transitions
• Callbacks: Execute logic on state changes
• Single Source of Truth: State stored in database

Implementation Guidelines

Basic State Machine

# Gemfile
gem 'aasm'

# app/models/order.rb
class Order < ApplicationRecord
    include AASM

    aasm column: :status do
        state :pending, initial: true
        state :confirmed, :shipped, :delivered, :cancelled

        event :confirm do
            transitions from: :pending, to: :confirmed
            after do
                # WHY: Update confirmation timestamp and send notification
                update!(confirmed_at: Time.current)
                OrderMailer.confirmation_email(self).deliver_later
            end
        end

        event :ship do
            transitions from: :confirmed, to: :shipped
            after do
                update!(shipped_at: Time.current)
                notify_shipping_service
            end
        end

        event :deliver do
            transitions from: :shipped, to: :delivered
            after do
                update!(delivered_at: Time.current)
            end
        end

        event :cancel do
            transitions from: [:pending, :confirmed], to: :cancelled
            after do
                refund_payment
                restore_inventory
            end
        end
    end

    private

    def notify_shipping_service
        # Shipping service integration
    end

    def refund_payment
        # Payment refund logic
    end

    def restore_inventory
        # Restore inventory logic
    end
end

Usage in Controllers

class OrdersController < ApplicationController
    def confirm
        @order = Order.find(params[:id])

        if @order.may_confirm?
            @order.confirm!
            redirect_to @order, notice: 'Order confirmed'
        else
            redirect_to @order, alert: 'Cannot confirm order'
        end
    rescue AASM::InvalidTransition => e
        redirect_to @order, alert: e.message
    end
end

Guards (Conditional Transitions)

class Order < ApplicationRecord
    include AASM

    aasm do
        state :pending, initial: true
        state :processing, :completed

        event :process do
            # WHY: Only allow processing if payment succeeded
            transitions from: :pending, to: :processing, guard: :payment_successful?
        end

        event :complete do
            # WHY: Require all items shipped before completion
            transitions from: :processing, to: :completed, guard: :all_items_shipped?
        end
    end

    def payment_successful?
        payment&.status == 'successful'
    end

    def all_items_shipped?
        order_items.all?(&:shipped?)
    end
end

Multiple Transitions for Same Event

class Document < ApplicationRecord
    include AASM

    aasm do
        state :draft, initial: true
        state :under_review, :approved, :rejected

        event :submit do
            # WHY: Route to review if complete, else stay in draft
            transitions from: :draft, to: :under_review, guard: :complete?
            transitions from: :draft, to: :draft, unless: :complete?

            after do
                notify_reviewers if under_review?
            end
        end

        event :review do
            transitions from: :under_review, to: :approved
            transitions from: :under_review, to: :rejected
        end
    end

    def complete?
        title.present? && content.present?
    end
end

Error Handling

class Order < ApplicationRecord
    include AASM

    aasm do
        state :pending, initial: true
        state :processing, :completed, :failed

        event :process do
            transitions from: :pending, to: :processing

            error do |e|
                # WHY: Log error and transition to failed state
                Rails.logger.error("Order processing failed: #{e.message}")
                self.error_message = e.message
                may_fail? ? fail! : raise
            end

            after do
                charge_payment
            end
        end

        event :fail do
            transitions from: :processing, to: :failed
        end
    end

    def charge_payment
        # May raise exception
        PaymentService.charge(self)
    end
end

State Scopes

class Order < ApplicationRecord
    include AASM

    aasm do
        state :pending, initial: true
        state :confirmed, :shipped, :delivered, :cancelled
        # ... events ...
    end

    # Automatically creates scopes
    # Order.pending
    # Order.confirmed
    # Order.shipped
end

# Usage
Order.confirmed.where(user: current_user)

State Queries

# Check current state
order.aasm_state  # => "confirmed"
order.pending?    # => false
order.confirmed?  # => true

# Check possible transitions
order.aasm.states(permitted: true)  # => [:shipped, :cancelled]

# Check if transition allowed
order.may_ship?      # => true
order.may_deliver?   # => false

# Get current event
order.aasm.current_event  # => :ship

Testing State Machines

# spec/models/order_spec.rb
RSpec.describe Order, type: :model do
    describe 'state machine' do
        let(:order) { create(:order) }

        it 'starts in pending state' do
            expect(order).to be_pending
        end

        describe '#confirm' do
            it 'transitions from pending to confirmed' do
                expect { order.confirm! }
                    .to change { order.aasm_state }
                    .from('pending').to('confirmed')
            end

            it 'updates confirmed_at timestamp' do
                expect { order.confirm! }
                    .to change { order.confirmed_at }
                    .from(nil)
            end

            it 'sends confirmation email' do
                expect(OrderMailer).to receive(:confirmation_email)
                order.confirm!
            end
        end

        describe '#ship' do
            it 'cannot ship pending order' do
                expect(order.may_ship?).to be false
            end

            it 'can ship confirmed order' do
                order.confirm!
                expect(order.may_ship?).to be true
            end
        end
    end
end

Tools to Use

• Read: Read existing state machine code
• Write: Create new state machine models
• Edit: Modify state machines
• Bash: Run tests and migrations

Bash Commands

# Add AASM gem
bundle add aasm

# Generate migration for state column
bundle exec rails generate migration AddStatusToOrders status:string

# Run tests
bundle exec rspec spec/models/order_spec.rb

Workflow

1. Identify States: Define all possible states
2. Define Events: List state transitions
3. Write Tests: Test each transition
4. Implement State Machine: Add AASM to model
5. Add Callbacks: Implement after/before logic
6. Add Guards: Implement conditional transitions
7. Test: Verify all transitions work

Related Skills

• rails-model-design: Understanding model structure
• rails-service-objects: Complex transition logic
• rails-background-jobs: Async operations after transitions
• rails-rspec-testing: Testing state machines

Coding Standards

See Rails Coding Standards

TDD Workflow

Follow TDD Workflow

Key Reminders

• Define initial state explicitly
• Use guards for conditional transitions
• Add callbacks for side effects
• Test all transitions and guards
• Use may_event? to check before transitioning
• Handle AASM::InvalidTransition exceptions
• Keep state machine logic in the model
• Use scopes for querying by state