Adds support for integrating state machines with DataMapper resources.

Examples

Below is an example of a simple state machine defined within a DataMapper resource:

  class Vehicle
    include DataMapper::Resource

    property :id, Serial
    property :name, String
    property :state, String

    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 resource to save the changes made to the state machine‘s attribute. Note that if any other changes were made to the resource 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 DataMapper, these automated events are run in the following order:

• before validation - If validation feature loaded, run before callbacks and persist new states, then validate
• before save - If validation feature was skipped/not loaded, 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                    # => #<DataMapper::Validate::ValidationErrors:0xb7a48b54 @errors={"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
    include DataMapper::Resource
    ...

    state_machine do
      ...
    end
    protected :state_event
  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 DataMapper::Resource
    ...

    state_machine do
      # Define private events here
    end
    protected :state_event= # Prevent access to events in the first machine

    # Allow both machines to share the same state
    state_machine :public_state, :attribute => :state do
      # Define public events here
    end
  end

Transactions

By default, the use of transactions during an event transition is turned off to be consistent with DataMapper. This means that if changes are made to the database during a before callback, but the transition fails to complete, those changes will not be rolled back.

For example,

  class Message
    include DataMapper::Resource

    property :id, Serial
    property :content, String
  end

  Vehicle.state_machine do
    before_transition do |transition|
      Message.create(:content => transition.inspect)
      throw :halt
    end
  end

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

To turn on transactions:

  class Vehicle
    include DataMapper::Resource
    ...

    state_machine :initial => :parked, :use_transactions => true do
      ...
    end
  end

If using the save action for the machine, this option will be ignored as the transaction behavior will depend on the save implementation within DataMapper. To avoid this, use a different action like so:

  class Vehicle
    include DataMapper::Resource
    ...

    state_machine :initial => :parked, :use_transactions => false, :action => :save_state do
      ...
    end

    alias_method :save_state, :save
  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                  # => ["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 class methods are defined on the model for finding records with or without a particular set of states.

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

  class Vehicle
    include DataMapper::Resource

    property :id, Serial
    property :state, String

    class << self
      def with_states(*states)
        all(:state => states.flatten)
      end
      alias_method :with_state, :with_states

      def without_states(*states)
        all(:state.not => states.flatten)
      end
      alias_method :without_state, :without_states
    end
  end

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

Because of the way scopes work in DataMapper, 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 / Observers

All before/after transition callbacks defined for DataMapper resources behave in the same way that other DataMapper hooks behave. Rather than passing in the record as an argument to the callback, the callback is instead bound to the object and evaluated within its context.

For example,

  class Vehicle
    include DataMapper::Resource

    property :id, Serial
    property :state, String

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

      before_transition do |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.

In addition to support for DataMapper-like hooks, there is additional support for DataMapper observers. See StateMachine::Integrations::DataMapper::Observer for more information.

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 DataMapper, a save failure will cause any records that get created in your callback to roll back. Note that this is only a problem if the machine is configured to use transactions. If it is, you can work around this issue like so:

  DataMapper.setup(:default, 'mysql://localhost/app')
  DataMapper.setup(:logs, 'mysql://localhost/app')

  class TransitionLog
    include DataMapper::Resource
  end

  class Vehicle < ActiveRecord::Base
    include DataMapper::Resource

    state_machine :use_transactions => true do
      after_failure do |transition|
        DataMapper.repository(:logs) do
          TransitionLog.create(:vehicle => vehicle, :transition => transition)
        end
      end

      ...
    end
  end

The failure callback creates TransitionLog records using a second connection to the database, allowing them to be saved without being affected by rollbacks in the Vehicle resource‘s transaction.