Module: StateMachines::Integrations::ActiveModel

Includes:
Base
Defined in:
lib/state_machines/integrations/active_model.rb,
lib/state_machines/integrations/active_model/version.rb

Overview

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::Validations

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

class Vehicle
  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

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 ActiveModel’s validation framework, custom validators will not work as expected when defined to run in multiple states. For example:

class Vehicle
  include ActiveModel::Validations

  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
  include ActiveModel::Validations

  state_machine do
    ...
    state :first_gear, :second_gear do
      validate {|vehicle| vehicle.speed_is_legal}
    end
  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\""]

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)

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
  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

Internationalization

Any error message that is generated from performing invalid transitions can be localized. The following default translations are used:

en:
  activemodel:
    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}"

You can override these for a specific model like so:

en:
  activemodel:
    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”:

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

  • activemodel.state_machines.#{model_name}.states.#{state_name}

  • activemodel.state_machines.#{machine_name}.states.#{state_name}

  • activemodel.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”:

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

  • activemodel.state_machines.#{model_name}.events.#{event_name}

  • activemodel.state_machines.#{machine_name}.events.#{event_name}

  • activemodel.state_machines.events.#{event_name}

An example translation configuration might look like so:

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

Dirty Attribute Tracking

When using the ActiveModel::Dirty extension, your model will keep track of any changes that are made to attributes. Depending on your ORM, an object will only be saved when there are attributes that have changed on the object. When integrating with state_machine, typically the state field will be marked as dirty after a transition occurs. In some situations, however, this isn’t the case.

If you define loopback transitions in your state machine, the value for the machine’s attribute (e.g. state) will not change. Unless you explicitly indicate so, this means that your object won’t persist anything on a loopback. For example:

class Vehicle
  include ActiveModel::Validations
  include ActiveModel::Dirty
  attr_accessor :state

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

If, instead, you’d like your object to always persist regardless of whether the value actually changed, you can do so by using the #{attribute}_will_change! helpers or defining a before_transition callback that actually changes an attribute on the model. For example:

class Vehicle
  ...
  state_machine :initial => :parked do
    before_transition all => same do |vehicle|
      vehicle.state_will_change!

      # Alternative solution, updating timestamp
      # vehicle.updated_at = Time.current
    end
  end
end

Creating new integrations

If you want to integrate state_machine with an ORM that implements parts or all of the ActiveModel API, only the machine defaults need to be specified. Otherwise, the implementation is similar to any other integration.

For example,

module StateMachine::Integrations::MyORM
  include ActiveModel

  mattr_accessor(:defaults) { :action => :persist }

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

  protected

  def runs_validations_on_action?
   action == :persist
  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.

Constant Summary collapse

VERSION = 
'0.8.0'

Class Method Summary collapse

Instance Method Summary collapse

Class Method Details

.matching_ancestors ⇒ Object

Classes that include ActiveModel::Validations will automatically use the ActiveModel integration.



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# File 'lib/state_machines/integrations/active_model.rb', line 369

def self.matching_ancestors
  [::ActiveModel, ::ActiveModel::Validations]
end

Instance Method Details

#around_validation(object) ⇒ Object

Runs state events around the object’s validation process



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# File 'lib/state_machines/integrations/active_model.rb', line 399

def around_validation(object)
  object.class.state_machines.transitions(object, action, after: false).perform { yield }
end

#errors_for(object) ⇒ Object

Describes the current validation errors on the given object. If none are specific, then the default error is interpeted as a “halt”.



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# File 'lib/state_machines/integrations/active_model.rb', line 389

def errors_for(object)
  object.errors.empty? ? 'Transition halted' : object.errors.full_messages * ', '
end

#invalidate(object, attribute, message, values = []) ⇒ Object

Adds a validation error to the given object



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# File 'lib/state_machines/integrations/active_model.rb', line 374

def invalidate(object, attribute, message, values = [])
  if supports_validations?
    attribute = self.attribute(attribute)
    options = values.reduce({}) do |h, (key, value)|
      h[key] = value
      h
    end

    default_options = default_error_message_options(object, attribute, message)
    object.errors.add(attribute, message, **options, **default_options)
  end
end

#reset(object) ⇒ Object

Resets any errors previously added when invalidating the given object



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# File 'lib/state_machines/integrations/active_model.rb', line 394

def reset(object)
  object.errors.clear if supports_validations?
end