Adds support for integrating state machines with Mongoid models.

Examples

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

  class Vehicle
    include Mongoid::Document

    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: 4d70e028b876bb54d9000003, name: nil, state: "parked">
  vehicle.name = 'Ford Explorer'
  vehicle.ignite                    # => true
  vehicle.reload                    # => #<Vehicle _id: 4d70e028b876bb54d9000003, 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 Mongoid, 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: 4d70e028b876bb54d9000003, 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: 4d70e028b876bb54d9000003, 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
    include Mongoid::Document

    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 Mongoid::Document

    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

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

  class Vehicle
    include Mongoid::Document

    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 Mongoid::Document

    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: 4d70e028b876bb54d9000003, 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 basic scopes are defined on the model for finding records with or without a particular set of states.

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

  class Vehicle
    include Mongoid::Document

    scope :with_states, lambda {|*states| where(:state => {'$in' => states})}
    # with_states also aliased to with_state

    scope :without_states, lambda {|*states| where(:state => {'$nin' => 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 Mongoid, they cannot be chained.

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 Mongoid models behave in the same way that other Mongoid callbacks behave. The object involved in the transition is passed in as an argument.

For example,

  class Vehicle
    include Mongoid::Document

    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 addition to support for Mongoid-like hooks, there is additional support for Mongoid observers. Because of the way Mongoid 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 < Mongoid::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 < Mongoid::Observer
    observe Vehicle, Switch, Project

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

Internationalization

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

  en:
    mongoid:
      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:
    mongoid:
      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":

• mongoid.state_machines.#{model_name}.#{machine_name}.states.#{state_name}
• mongoid.state_machines.#{model_name}.states.#{state_name}
• mongoid.state_machines.#{machine_name}.states.#{state_name}
• mongoid.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":

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

An example translation configuration might look like so:

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