Adds support for integrating state machines with ActiveModel classes.

Examples

If using ActiveModel directly within your class, then any one of the following features need to be included in order for the integration to be detected:

• ActiveModel::Dirty
• ActiveModel::Observing
• ActiveModel::Validations

Below is an example of a simple state machine defined within an ActiveModel class:

  class Vehicle
    include ActiveModel::Dirty
    include ActiveModel::Observing
    include ActiveModel::Validations

    attr_accessor :state
    define_attribute_methods [:state]

    state_machine :initial => :parked do
      event :ignite do
        transition :parked => :idling
      end
    end
  end

The examples in the sections below will use the above class as a reference.

Actions

By default, no action will be invoked when a state is transitioned. This means that if you want to save changes when transitioning, you must define the action yourself like so:

  class Vehicle
    include ActiveModel::Validations
    attr_accessor :state

    state_machine :action => :save do
      ...
    end

    def save
      # Save changes
    end
  end

Validation errors

In order to hook in validation support for your model, the ActiveModel::Validations feature must be included. If this is included and an event fails to successfully fire because there are no matching transitions for the object, a validation error is added to the object‘s state attribute to help in determining why it failed.

For example,

  vehicle = Vehicle.new
  vehicle.ignite                # => false
  vehicle.errors.full_messages  # => ["State cannot transition via \"ignite\""]

Security implications

Beware that public event attributes mean that events can be fired whenever mass-assignment is being used. If you want to prevent malicious users from tampering with events through URLs / forms, the attribute should be protected like so:

  class Vehicle
    include ActiveModel::MassAssignmentSecurity
    attr_accessor :state

    attr_protected :state_event
    # attr_accessible ... # Alternative technique

    state_machine do
      ...
    end
  end

If you want to only have some events be able to fire via mass-assignment, you can build two state machines (one public and one protected) like so:

  class Vehicle
    include ActiveModel::MassAssignmentSecurity
    attr_accessor :state

    attr_protected :state_event # Prevent access to events in the first machine

    state_machine do
      # Define private events here
    end

    # Public machine targets the same state as the private machine
    state_machine :public_state, :attribute => :state do
      # Define public events here
    end
  end

Callbacks

All before/after transition callbacks defined for ActiveModel models behave in the same way that other ActiveSupport callbacks behave. The object involved in the transition is passed in as an argument.

For example,

  class Vehicle
    include ActiveModel::Validations
    attr_accessor :state

    state_machine :initial => :parked do
      before_transition any => :idling do |vehicle|
        vehicle.put_on_seatbelt
      end

      before_transition do |vehicle, transition|
        # log message
      end

      event :ignite do
        transition :parked => :idling
      end
    end

    def put_on_seatbelt
      ...
    end
  end

Note, also, that the transition can be accessed by simply defining additional arguments in the callback block.

Observers

In order to hook in observer support for your application, the ActiveModel::Observing feature must be included. Because of the way ActiveModel observers are designed, there is less flexibility around the specific transitions that can be hooked in. However, a large number of hooks are supported. For example, if a transition for a object‘s state attribute changes the state from parked to idling via the ignite event, the following observer methods are supported:

• before/after/after_failure_to-_ignite_from_parked_to_idling
• before/after/after_failure_to-_ignite_from_parked
• before/after/after_failure_to-_ignite_to_idling
• before/after/after_failure_to-_ignite
• before/after/after_failure_to-_transition_state_from_parked_to_idling
• before/after/after_failure_to-_transition_state_from_parked
• before/after/after_failure_to-_transition_state_to_idling
• before/after/after_failure_to-_transition_state
• before/after/after_failure_to-_transition

The following class shows an example of some of these hooks:

  class VehicleObserver < ActiveModel::Observer
    # Callback for :ignite event *before* the transition is performed
    def before_ignite(vehicle, transition)
      # log message
    end

    # Callback for :ignite event *after* the transition has been performed
    def after_ignite(vehicle, transition)
      # put on seatbelt
    end

    # Generic transition callback *before* the transition is performed
    def after_transition(vehicle, transition)
      Audit.log(vehicle, transition)
    end

    def after_failure_to_transition(vehicle, transition)
      Audit.error(vehicle, transition)
    end
  end

More flexible transition callbacks can be defined directly within the model as described in StateMachine::Machine#before_transition and StateMachine::Machine#after_transition.

To define a single observer for multiple state machines:

  class StateMachineObserver < ActiveModel::Observer
    observe Vehicle, Switch, Project

    def after_transition(object, transition)
      Audit.log(object, transition)
    end
  end

Dirty Attribute Tracking

In order to hook in validation support for your model, the ActiveModel::Validations feature must be included. If this is included then state attributes will always be properly marked as changed whether they were a callback or not.

For example,

  class Vehicle
    include ActiveModel::Dirty
    attr_accessor :state

    state_machine :initial => :parked do
      event :park do
        transition :parked => :parked
      end
    end
  end

  vehicle = Vehicle.new
  vehicle.changed         # => []
  vehicle.park            # => true
  vehicle.changed         # => ["state"]

Creating new integrations

If you want to integrate state_machine with an ORM that implements parts or all of the ActiveModel API, the following features must be specified:

• i18n scope (locale)
• Machine defaults

For example,

  module StateMachine::Integrations::MyORM
    include StateMachine::Integrations::ActiveModel

    @defaults = {:action = > :persist}

    def self.matches?(klass)
      defined?(::MyORM::Base) && klass <= ::MyORM::Base
    end

    def self.extended(base)
      locale = "#{File.dirname(__FILE__)}/my_orm/locale.rb"
      I18n.load_path << locale unless I18n.load_path.include?(locale)
    end

    protected
      def runs_validations_on_action?
        action == :persist
      end

      def i18n_scope
        :myorm
      end
  end

If you wish to implement other features, such as attribute initialization with protected attributes, named scopes, or database transactions, you must add these independent of the ActiveModel integration. See the ActiveRecord implementation for examples of these customizations.