RailsMachine

A small state machine for ActiveRecord models, built on top of Rails enums.

Add to your Gemfile:

gem 'rails_machine'

Requires Ruby >= 3.0 and ActiveRecord >= 7.0.

Add an integer column to your model to hold the state:

class AddStateToVehicles < ActiveRecord::Migration[7.0]
  def change
    add_column :vehicles, :state, :integer, default: 0, null: false
  end
end

Include RailsMachine and declare the machine:

class Vehicle < ActiveRecord::Base
  include RailsMachine

  rails_machine do
    state :stopped
    state :idling
    state :driving
    state :broken

    init_state :stopped

    transition from: :stopped, to: :idling
    transition from: :idling,  to: :stopped
    transition from: :idling,  to: :driving
    transition from: :driving, to: :idling

    transition from: :any,    to: :broken
    transition from: :broken, to: :stopped, guards: [->(v) { v.inspected? }]
  end
end

States are stored as Rails enums. IDs start at 0 and increment by 1, or you can set them explicitly:

state :stopped           # id 0
state :idling            # id 1
state :archived, id: 99  # id 99
state :legacy            # id 100

You get all the usual enum helpers for free:

vehicle.stopped?      # => true
vehicle.idling!       # persists the transition
Vehicle.stopped       # scope
Vehicle.states        # { "stopped" => 0, "idling" => 1, ... }

Defining the same state name twice raises ArgumentError.

init_state declares which states a record is allowed to start in. Records saved with any other initial value fail validation.

init_state :stopped
init_state :broken   # multiple allowed

If no init_state is declared, any state is accepted on creation.

Each transition defines a legal from → to pair. Attempting an undeclared transition raises ActiveRecord::RecordInvalid on save (or returns false from valid?).

:any is a wildcard:

transition from: :any, to: :broken      # any state can become :broken
transition from: :broken, to: :any      # :broken can become anything
transition from: :any, to: :any         # no restrictions

Guards are callables that receive the record and must return truthy for the transition to pass:

transition from: :broken, to: :stopped, guards: [->(v) { v.inspected? }]

If every guard on every matching transition returns falsy, validation adds a :guard_failed error.

Pass column: to use something other than :state:

rails_machine column: :status do
  state :draft
  state :published
  transition from: :draft, to: :published
end

Validation failures add errors on the state column with these keys:

• :invalid_init_state — record was created in a state not listed in init_state

• :transition_not_found — no transition defined for the attempted state change

• :guard_failed — a matching transition exists but its guard(s) returned falsy

Default English messages ship with the gem. Override them under en.errors.messages in your own locale files.

MIT.