Adds support for integrating state machines with ActiveRecord models.

Examples

Below is an example of a simple state machine defined within an ActiveRecord model:

  class Vehicle < ActiveRecord::Base
    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, the action that will be invoked when a state is transitioned is the save action. This will cause the record to save the changes made to the state machine‘s attribute. Note that if any other changes were made to the record prior to transition, then those changes will be saved as well.

For example,

  vehicle = Vehicle.create          # => #<Vehicle id: 1, name: nil, state: "parked">
  vehicle.name = 'Ford Explorer'
  vehicle.ignite                    # => true
  vehicle.reload                    # => #<Vehicle id: 1, name: "Ford Explorer", state: "idling">

Events

As described in StateMachine::InstanceMethods#state_machine, event attributes are created for every machine that allow transitions to be performed automatically when the object‘s action (in this case, :save) is called.

In ActiveRecord, these automated events are run in the following order:

• before validation - Run before callbacks and persist new states, then validate
• before save - If validation was skipped, run before callbacks and persist new states, then save
• after save - Run after callbacks

For example,

  vehicle = Vehicle.create          # => #<Vehicle id: 1, name: nil, state: "parked">
  vehicle.state_event               # => nil
  vehicle.state_event = 'invalid'
  vehicle.valid?                    # => false
  vehicle.errors.full_messages      # => ["State event is invalid"]

  vehicle.state_event = 'ignite'
  vehicle.valid?                    # => true
  vehicle.save                      # => true
  vehicle.state                     # => "idling"
  vehicle.state_event               # => nil

Note that this can also be done on a mass-assignment basis:

  vehicle = Vehicle.create(:state_event => 'ignite')  # => #<Vehicle id: 1, name: nil, state: "idling">
  vehicle.state                                       # => "idling"

This technique is always used for transitioning states when the save action (which is the default) is configured for the machine.

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 < ActiveRecord::Base
    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 < ActiveRecord::Base
    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

Transactions

In order to ensure that any changes made during transition callbacks are rolled back during a failed attempt, every transition is wrapped within a transaction.

For example,

  class Message < ActiveRecord::Base
  end

  Vehicle.state_machine do
    before_transition do |vehicle, transition|
      Message.create(:content => transition.inspect)
      false
    end
  end

  vehicle = Vehicle.create      # => #<Vehicle id: 1, name: nil, state: "parked">
  vehicle.ignite                # => false
  Message.count                 # => 0

Note that only before callbacks that halt the callback chain and failed attempts to save the record will result in the transaction being rolled back. If an after callback halts the chain, the previous result still applies and the transaction is not rolled back.

To turn off transactions:

  class Vehicle < ActiveRecord::Base
    state_machine :initial => :parked, :use_transactions => false do
      ...
    end
  end

If using the save action for the machine, this option will be ignored as the transaction will be created by ActiveRecord within save. To avoid this, use a different action like so:

  class Vehicle < ActiveRecord::Base
    state_machine :initial => :parked, :use_transactions => false, :action => :save_state do
      ...
    end

    alias_method :save_state, :save
  end

Validations

As mentioned in StateMachine::Machine#state, you can define behaviors, like validations, that only execute for certain states. One important caveat here is that, due to a constraint in ActiveRecord‘s validation framework, custom validators will not work as expected when defined to run in multiple states. For example:

  class Vehicle < ActiveRecord::Base
    state_machine do
      ...
      state :first_gear, :second_gear do
        validate :speed_is_legal
      end
    end
  end

In this case, the :speed_is_legal validation will only get run for the :second_gear state. To avoid this, you can define your custom validation like so:

  class Vehicle < ActiveRecord::Base
    state_machine do
      ...
      state :first_gear, :second_gear do
        validate {|vehicle| vehicle.speed_is_legal}
      end
    end
  end

Validation errors

If an event fails to successfully fire because there are no matching transitions for the current record, a validation error is added to the record‘s state attribute to help in determining why it failed and for reporting via the UI.

For example,

  vehicle = Vehicle.create(:state => 'idling')  # => #<Vehicle id: 1, name: nil, state: "idling">
  vehicle.ignite                                # => false
  vehicle.errors.full_messages                  # => ["State cannot transition via \"ignite\""]

If an event fails to fire because of a validation error on the record and not because a matching transition was not available, no error messages will be added to the state attribute.

In addition, if you‘re using the ignite! version of the event, then the failure reason (such as the current validation errors) will be included in the exception that gets raised when the event fails. For example, assuming there‘s a validation on a field called name on the class:

  vehicle = Vehicle.new
  vehicle.ignite!       # => StateMachine::InvalidTransition: Cannot transition state via :ignite from :parked (Reason(s): Name cannot be blank)

Scopes

To assist in filtering models with specific states, a series of named scopes are defined on the model for finding records with or without a particular set of states.

These named scopes are essentially the functional equivalent of the following definitions:

  class Vehicle < ActiveRecord::Base
    named_scope :with_states, lambda {|*states| {:conditions => {:state => states}}}
    # with_states also aliased to with_state

    named_scope :without_states, lambda {|*states| {:conditions => ['state NOT IN (?)', states]}}
    # without_states also aliased to without_state
  end

Note, however, that the states are converted to their stored values before being passed into the query.

Because of the way named scopes work in ActiveRecord, they can be chained like so:

  Vehicle.with_state(:parked).all(:order => 'id DESC')

Note that states can also be referenced by the string version of their name:

  Vehicle.with_state('parked')

Callbacks

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

For example,

  class Vehicle < ActiveRecord::Base
    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.

Failure callbacks

after_failure callbacks allow you to execute behaviors when a transition is allowed, but fails to save. This could be useful for something like auditing transition attempts. Since callbacks run within transactions in ActiveRecord, a save failure will cause any records that get created in your callback to roll back. You can work around this issue like so:

  class TransitionLog < ActiveRecord::Base
    establish_connection Rails.env.to_sym
  end

  class Vehicle < ActiveRecord::Base
    state_machine do
      after_failure do |vehicle, transition|
        TransitionLog.create(:vehicle => vehicle, :transition => transition)
      end

      ...
    end
  end

The TransitionLog model establishes a second connection to the database that allows new records to be saved without being affected by rollbacks in the Vehicle model‘s transaction.

Observers

In addition to support for ActiveRecord-like hooks, there is additional support for ActiveRecord observers. Because of the way ActiveRecord 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 record‘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 < ActiveRecord::Observer
    def before_save(vehicle)
      # log message
    end

    # 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
  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 < ActiveRecord::Observer
    observe Vehicle, Switch, Project

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

Internationalization

In Rails 2.2+, any error message that is generated from performing invalid transitions can be localized. The following default translations are used:

  en:
    activerecord:
      errors:
        messages:
          invalid: "is invalid"
          # %{value} = attribute value, %{state} = Human state name
          invalid_event: "cannot transition when %{state}"
          # %{value} = attribute value, %{event} = Human event name, %{state} = Human current state name
          invalid_transition: "cannot transition via %{event}"

Notice that the interpolation syntax is %{key} in Rails 3+. In Rails 2.x, the appropriate syntax is {{key}}.

You can override these for a specific model like so:

  en:
    activerecord:
      errors:
        models:
          user:
            invalid: "is not valid"

In addition to the above, you can also provide translations for the various states / events in each state machine. Using the Vehicle example, state translations will be looked for using the following keys, where model_name = "vehicle", machine_name = "state" and state_name = "parked":

• activerecord.state_machines.#{model_name}.#{machine_name}.states.#{state_name}
• activerecord.state_machines.#{model_name}.states.#{state_name}
• activerecord.state_machines.#{machine_name}.states.#{state_name}
• activerecord.state_machines.states.#{state_name}

Event translations will be looked for using the following keys, where model_name = "vehicle", machine_name = "state" and event_name = "ignite":

• activerecord.state_machines.#{model_name}.#{machine_name}.events.#{event_name}
• activerecord.state_machines.#{model_name}.events.#{event_name}
• activerecord.state_machines.#{machine_name}.events.#{event_name}
• activerecord.state_machines.events.#{event_name}

An example translation configuration might look like so:

  es:
    activerecord:
      state_machines:
        states:
          parked: 'estacionado'
        events:
          park: 'estacionarse'