Module: StateMachine::Integrations::DataMapper

Includes:
Base
Defined in:
lib/state_machine/integrations/data_mapper.rb,
lib/state_machine/integrations/data_mapper/observer.rb,
lib/state_machine/integrations/data_mapper/versions.rb

Overview

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

Within DataMapper Hooks

DataMapper protects against the potential for system stack errors resulting from infinite loops by preventing records from being saved multiple times within save hooks. You need to be acutely aware of this when interacting with state_machine within save hooks. There are two things to keep in mind:

  1. You cannot run a state_machine event during an ‘after :save/:create` hook.

  2. If you need to run a state_machine event during a ‘before :save/:create/etc.` hook, then you have to force the machine’s action to be skipped by passing ‘false` in as an argument to the event.

For example:

class Vehicle
  include DataMapper::Resource
  ...

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

  # This will allow the event to transition without attempting to save a second time
  before :create { ignite(false) }

  # This will never work because DataMapper will refuse to save the
  # changes since we're still inside of a transaction
  # after :create { ignite }
end

While the above will work, in reality you should typically just set the ‘state_event` attribute in `#initialize` to automatically transition an object on creation.

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

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.

Callback Order

Callbacks occur in the following order. Callbacks specific to state_machine are bolded. The remaining callbacks are part of ActiveRecord.

  • (-) save

  • (-) begin transaction (if enabled)

  • (1) before_transition

  • (2) before :valid?

  • (-) valid?

  • (3) after :valid?

  • (4) before :save

  • (-) save

  • (5) before :create

  • (-) create

  • (6) after :create

  • (7) after :save

  • (8) after_transition

  • (-) end transaction (if enabled)

Defined Under Namespace

Modules: Observer

Instance Attribute Summary

Attributes included from Base::ClassMethods

#defaults

Class Method Summary collapse

Instance Method Summary collapse

Methods included from Base

included

Methods included from Base::ClassMethods

#available?, #extended, #integration_name, #locale_path, #matches?, #matches_ancestors?, #matching_ancestors, #version, #versions

Class Method Details

.extended(base) ⇒ Object

Loads additional files specific to DataMapper



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# File 'lib/state_machine/integrations/data_mapper.rb', line 361

def self.extended(base) #:nodoc:
  require 'dm-core/version' unless ::DataMapper.const_defined?('VERSION')
  super
end

.matching_ancestors ⇒ Object

Classes that include DataMapper::Resource will automatically use the DataMapper integration.



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# File 'lib/state_machine/integrations/data_mapper.rb', line 356

def self.matching_ancestors
  %w(DataMapper::Resource)
end

Instance Method Details

#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_machine/integrations/data_mapper.rb', line 373

def errors_for(object)
  if object.errors.empty?
    'Transition halted'
  else
    errors = []
    object.errors.each_pair do |field_name, field_errors|
      field_errors.each {|error| errors << "#{field_name} #{error}"}
    end
    errors * ', '
  end
end

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

Adds a validation error to the given object



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# File 'lib/state_machine/integrations/data_mapper.rb', line 367

def invalidate(object, attribute, message, values = [])
  object.errors.add(self.attribute(attribute), generate_message(message, values)) if supports_validations?
end

#reset(object) ⇒ Object

Resets any errors previously added when invalidating the given object



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# File 'lib/state_machine/integrations/data_mapper.rb', line 386

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