Module StateMachine::Integrations::Sequel
In: lib/state_machine/integrations/sequel/versions.rb
lib/state_machine/integrations/sequel.rb

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.

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.

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)

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.

Methods

Included Modules

Base

Attributes

defaults  [R] 

Public Class methods

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

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

Public Instance methods

Adds a validation error to the given object

Pluralizes the name using the built-in inflector

Resets any errors previously added when invalidating the given object

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

Protected Instance methods

Uses internal save hooks if using the :save action

Creates a new callback in the callback chain, always ensuring that it‘s configured to bind to the object as this is the convention for Sequel callbacks

Creates a scope for finding records with a particular state or states for the attribute

Creates a scope for finding records without a particular state or states for the attribute

Adds hooks into validation for automatically firing events. This is a bit more complicated than other integrations since Sequel doesn‘t provide an easy way to hook around validation calls

Uses custom hooks for :save actions in order to preserve failure behavior within Sequel. This is a bit more complicated than other integrations since Sequel doesn‘t provide an easy way to hook around save calls.

Skips defining reader/writer methods since this is done automatically

Defines an initialization hook into the owner class for setting the initial state of the machine before any attributes are set on the object

Handles how save failures are raised

Handles whether validation errors should be raised

Loads the built-in inflector

Runs a new database transaction, rolling back any changes if the yielded block fails (i.e. returns false).

[Validate]