This document discusses using the AASM gem to implement finite state machines in Ruby. It shows how to define a state machine on a service object called AccountManager to manage the step-by-step creation of an Account model. The AccountManager handles validation and state transitions between steps 1, 2, 3 and finished. This separates the state machine logic from the Account model for better structure, maintenance and testability compared to defining the state machine directly on the Account model.

1. USING AASM IN SERVICE
OBJECT
Kamil BaćkowskiKamil Baćkowski

2. WHAT IS AASM ?
Library for adding finite state machines to RubyLibrary for adding finite state machines to Ruby
classes.classes.

3. EXAMPLE USAGE
class Job
include AASM
aasm do
state :sleeping, :initial => true
state :running, :cleaning
event :run do
transitions :from => :sleeping, :to => :running
end
event :clean do
transitions :from => :running, :to => :cleaning
end
end
end

4. OUR SCENARIO
class Account < ApplicationRecord
enum state: { step1: 'step1', step2: 'step2',
step3: 'step3', finished: 'finished' }
end

5. Service objectService object
class AccountManager
include AASM, ActiveModel::Model
attr_accessor :account, :name, :country, :language
validates :name, presence: true, if: :step1?
validates :country, presence: true, if: :step2?
validates :language, presence: true, if: :step3?
aasm do
state :step1, initial: true
state :step2, :step3, :finished
after_all_transitions :update_account_state
event :next_step do
transitions from: :step1, to: :step2
transitions from: :step2, to: :step3
transitions from: :step3, to: :finished
end
end
def initialize(attributes = {})
super
aasm_write_state_without_persistence(account.state.to_sym)
end
def save
if valid?
account.update! send(STEP_AVAILABLE_VALUES.fetch(aasm.current_state))
else
false
end
end
private
def update_account_state
account.update_column(:state, aasm.to_state)
end
end

6. CONTROLLER
class AccountsController < ApplicationController
def new
@account = Account.new
end
def edit
@account = Account.find(params[:id])
end
def create
@account = Account.new
account_manager = AccountManager.new(account_params.merge(account: @account))
redirect_to edit_account_path(@account) if account_manager.save && account_manager.next_step
end
def update
@account = Account.find(params[:id])
account_manager = AccountManager.new(account_params.merge(account: @account))
redirect_to edit_account_path(@account) if account_manager.save && account_manager.next_step
end
private
def account_params
params.require(:account).permit(:name, :country, :language)
end
end

7. ADVANTAGES
Proper logic separation
Easy to maintain
Simple to unit test
Can bypass aasm transitions in tests

8. SUMMARY
Always use aasm oustide of ActiveRecord models.Always use aasm oustide of ActiveRecord models.
Questions ?Questions ?