Module: StateMachine::Integrations::Sequel

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

Overview

Adds support for integrating state machines with Sequel models.

Examples

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

class Vehicle < Sequel::Model
  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 made as well.

For example,

vehicle = Vehicle.create          # => #<Vehicle @values={:state=>"parked", :name=>nil, :id=>1}>
vehicle.name = 'Ford Explorer'
vehicle.ignite                    # => true
vehicle.refresh                   # => #<Vehicle @values={:state=>"idling", :name=>"Ford Explorer", :id=>1}>

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 Sequel, 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 @values={:state=>"parked", :name=>nil, :id=>1}>
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                      # => #<Vehicle @values={:state=>"idling", :name=>nil, :id=>1}>
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 @values={:state=>"idling", :name=>nil, :id=>1}>
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 < Sequel::Model
  set_restricted_columns :state_event
  # set_allowed_columns ... # 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 < Sequel::Model
  set_restricted_columns :state_event # Prevent access to events in the first machine

  state_machine do
    # Define private events here
  end

  # Allow both machines to share the same state
  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 < Sequel::Model
end

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

vehicle = Vehicle.create      # => #<Vehicle @values={:state=>"parked", :name=>nil, :id=>1}>
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 < Sequel::Model
  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 Sequel within save. To avoid this, use a different action like so:

class Vehicle < Sequel::Model
  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 @values={:state=>"parked", :name=>nil, :id=>1}>
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 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 < Sequel::Model
  class << self
    def with_states(*states)
      filter(:state => states)
    end
    alias_method :with_state, :with_states

    def without_states(*states)
      filter(~{:state => states})
    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 Sequel, they can be chained like so:

Vehicle.with_state(:parked).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 Sequel resources behave in the same way that other Sequel 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 < Sequel::Model
  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.

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 Sequel, a save failure will cause any records that get created in your callback to roll back. You can work around this issue like so:

DB = Sequel.connect('mysql://localhost/app')
DB_LOGS = Sequel.connect('mysql://localhost/app')

class TransitionLog < Sequel::Model(DB_LOGS[:transition_logs])
end

class Vehicle < Sequel::Model(DB[:vehicles])
  state_machine do
    after_failure do |transition|
      TransitionLog.create(:vehicle => vehicle, :transition => transition)
    end

    ...
  end
end

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

Class Attribute Summary (collapse)

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?, #version, #versions

Class Attribute Details

+ (Object) defaults (readonly)

Returns the value of attribute defaults



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# File 'lib/state_machine/integrations/sequel.rb', line 274

def defaults
  @defaults
end

Class Method Details

+ (Boolean) available?

Whether this integration is available. Only true if Sequel::Model is defined.



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# File 'lib/state_machine/integrations/sequel.rb', line 279

def self.available?
  defined?(::Sequel::Model)
end

+ (Boolean) matches?(klass)

Should this integration be used for state machines in the given class? Classes that include Sequel::Model will automatically use the Sequel integration.



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# File 'lib/state_machine/integrations/sequel.rb', line 286

def self.matches?(klass)
  klass <= ::Sequel::Model
end

Instance Method Details

- (Object) errors_for(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/sequel.rb', line 310

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

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

Adds a validation error to the given object



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# File 'lib/state_machine/integrations/sequel.rb', line 304

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

- (Object) pluralize(word)

Pluralizes the name using the built-in inflector



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# File 'lib/state_machine/integrations/sequel.rb', line 320

def pluralize(word)
  load_inflector
  super
end

- (Object) reset(object)

Resets any errors previously added when invalidating the given object



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# File 'lib/state_machine/integrations/sequel.rb', line 315

def reset(object)
  object.errors.clear
end

- (Object) write(object, attribute, value, *args)

Forces the change in state to be recognized regardless of whether the state value actually changed



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# File 'lib/state_machine/integrations/sequel.rb', line 292

def write(object, attribute, value, *args)
  result = super
  
  column = self.attribute.to_sym
  if (attribute == :state || attribute == :event && value) && owner_class.columns.include?(column) && !object.changed_columns.include?(column)
    object.changed_columns << column
  end
  
  result
end