Module: StateMachines::Integrations::ActiveRecord

Includes:
ActiveModel, Base
Defined in:
lib/state_machines/integrations/active_record.rb,
lib/state_machines/integrations/active_record/version.rb

Overview

Adds support for integrating state machines with ActiveRecord models.

Examples

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

class Vehicle < ApplicationRecord
  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: 1, name: nil, state: "parked">
vehicle.name = 'Ford Explorer'
vehicle.ignite                    # => true
vehicle.reload                    # => #<Vehicle id: 1, name: "Ford Explorer", state: "idling">

Note that if you want a transition to update additional attributes of the record, either the changes need to be made in a before_transition callback or you need to save the record manually.

Events

As described in StateMachines::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 ActiveRecord, 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: 1, 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: 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, you should use Rails’ strong parameters to control which attributes are permitted:

class VehiclesController < ApplicationController
  def vehicle_params
    params.require(:vehicle).permit(:color, :make, :model)
    # Exclude state_event to prevent tampering
  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 < ApplicationRecord
  # Define private 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

  # Control access via strong parameters in your controller
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 < ApplicationRecord
end

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

vehicle = Vehicle.create      # => #<Vehicle id: 1, name: nil, state: "parked">
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 < ApplicationRecord
  state_machine :initial => :parked, :use_transactions => false do
    ...
  end
end

Validations

As mentioned in StateMachines::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 ActiveRecord’s validation framework, custom validators will not work as expected when defined to run in multiple states. For example:

class Vehicle < ApplicationRecord
  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 < ApplicationRecord
  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: 1, 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!       # => StateMachines::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 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 < ApplicationRecord
  # with_states also aliased to with_state
  scope :with_states, ->(states) { where(state: states) }

  # without_states also aliased to without_state
  scope :without_states, ->(states) { where.not(state: states) }
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 ActiveRecord, 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 ActiveRecord models behave in the same way that other ActiveRecord callbacks behave. The object involved in the transition is passed in as an argument.

For example,

class Vehicle < ApplicationRecord
  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.

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

class TransitionLog < ApplicationRecord
  connects_to database: { writing: :primary, reading: :primary }
end

class Vehicle < ApplicationRecord
  state_machine do
    after_failure do |vehicle, transition|
      TransitionLog.create(:vehicle => vehicle, :transition => transition)
    end

    ...
  end
end

The TransitionLog model establishes a separate connection to the database that allows new records to be saved without being affected by rollbacks in the Vehicle model’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

  • (-) valid

  • (2) before_validation

  • (-) validate

  • (3) after_validation

  • (4) before_save

  • (5) before_create

  • (-) create

  • (6) after_create

  • (7) after_save

  • (8) after_transition

  • (-) end transaction (if enabled)

  • (9) after_commit

Internationalization

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

en:
  activerecord:
    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:
  activerecord:
    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”:

  • activerecord.state_machines.#{model_name}.#{machine_name}.states.#{state_name}

  • activerecord.state_machines.#{model_name}.states.#{state_name}

  • activerecord.state_machines.#{machine_name}.states.#{state_name}

  • activerecord.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”:

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

  • activerecord.state_machines.#{model_name}.events.#{event_name}

  • activerecord.state_machines.#{machine_name}.events.#{event_name}

  • activerecord.state_machines.events.#{event_name}

An example translation configuration might look like so:

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

Constant Summary collapse

VERSION = 
'0.9.0'

Class Method Summary collapse

Class Method Details

.matching_ancestors ⇒ Object

Classes that inherit from ActiveRecord::Base will automatically use the ActiveRecord integration.



365
366
367
 
# File 'lib/state_machines/integrations/active_record.rb', line 365

def matching_ancestors
  [::ActiveRecord::Base]
end