AASM - Ruby state machines

This package contains AASM, a library for adding finite state machines to Ruby classes.

AASM started as the acts_as_state_machine plugin but has evolved into a more generic library that no longer targets only ActiveRecord models. It currently provides adapters for ActiveRecord and Mongoid, but it can be used for any Ruby class, no matter what parent class it has (if any).

Usage

Adding a state machine is as simple as including the AASM module and start defining states and events together with their transitions:

class Job
  include AASM

  aasm do
    state :sleeping, :initial => true
    state :running
    state :cleaning

    event :run do
      transitions :from => :sleeping, :to => :running
    end

    event :clean do
      transitions :from => :running, :to => :cleaning
    end

    event :sleep do
      transitions :from => [:running, :cleaning], :to => :sleeping
    end
  end

end

This provides you with a couple of public methods for instances of the class Job:

job = Job.new
job.sleeping? # => true
job.may_run?  # => true
job.run
job.running?  # => true
job.sleeping? # => false
job.may_run?  # => false
job.run       # => raises AASM::InvalidTransition

If you don't like exceptions and prefer a simple true or false as response, tell AASM not to be whiny:

class Job
  ...
  aasm :whiny_transitions => false do
    ...
  end
end

job.running?  # => true
job.may_run?  # => false
job.run       # => false

When firing an event, you can pass a block to the method, it will be called only if the transition succeeds :

  job.run do
    job.user.notify_job_ran # Will be called if job.may_run? is true
  end

Callbacks

You can define a number of callbacks for your transitions. These methods will be called, when certain criteria are met, like entering a particular state:

class Job
  include AASM

  aasm do
    state :sleeping, :initial => true, :before_enter => :do_something
    state :running

    event :run, :after => Proc.new { do_afterwards } do
      transitions :from => :sleeping, :to => :running, :on_transition => Proc.new {|obj, *args| obj.set_process(*args) }
    end

    event :sleep do
      after do
        ...
      end
      error do |e|
        ...
      end
      transitions :from => :running, :to => :sleeping
    end
  end

  def set_process(name)
    ...
  end

  def do_something
    ...
  end

  def do_afterwards
    ...
  end

end

In this case do_something is called before actually entering the state sleeping, while do_afterwards is called after the transition run (from sleeping to running) is finished.

Here you can see a list of all possible callbacks, together with their order of calling:

  event:before
    previous_state:before_exit
      new_state:before_enter
        ...update state...
      previous_state:after_exit
    new_state:after_enter
  event:after

Also, you can pass parameters to events:

  job = Job.new
  job.run(:running, :defragmentation)

In this case the set_process would be called with :defagmentation argument.

In case of an error during the event processing the error is rescued and passed to :error callback, which can handle it or re-raise it for further propagation.

During the :on_transition callback (and reliably only then) you can access the originating state (the from-state) and the target state (the to state), like this:

  def set_process(name)
    logger.info "from #{aasm.from_state} to #{aasm.to_state}"
  end

Guards

Let's assume you want to allow particular transitions only if a defined condition is given. For this you can set up a guard per transition, which will run before actually running the transition. If the guard returns false the transition will be denied (raising AASM::InvalidTransition or returning false itself):

class Job
  include AASM

  aasm do
    state :sleeping, :initial => true
    state :running
    state :cleaning

    event :run do
      transitions :from => :sleeping, :to => :running
    end

    event :clean do
      transitions :from => :running, :to => :cleaning
    end

    event :sleep do
      transitions :from => :running, :to => :sleeping, :guard => :cleaning_needed?
    end
  end

  def cleaning_needed?
    false
  end

end

job = Job.new
job.run
job.may_sleep?  # => false
job.sleep       # => raises AASM::InvalidTransition

You can even provide a number of guards, which all have to succeed to proceed

    def walked_the_dog?; ...; end

    event :sleep do
      transitions :from => :running, :to => :sleeping, :guards => [:cleaning_needed?, :walked_the_dog?]
    end

If you want to provide guards for all transitions withing an event, you can use event guards

    event :sleep, :guards => [:walked_the_dog?] do
      transitions :from => :running, :to => :sleeping, :guards => [:cleaning_needed?]
      transitions :from => :cleaning, :to => :sleeping
    end

ActiveRecord

AASM comes with support for ActiveRecord and allows automatical persisting of the object's state in the database.

class Job < ActiveRecord::Base
  include AASM

  aasm do # default column: aasm_state
    state :sleeping, :initial => true
    state :running

    event :run do
      transitions :from => :sleeping, :to => :running
    end

    event :sleep do
      transitions :from => :running, :to => :sleeping
    end
  end

end

You can tell AASM to auto-save the object or leave it unsaved

job = Job.new
job.run   # not saved
job.run!  # saved

Saving includes running all validations on the Job class. If you want make sure the state gets saved without running validations (and thereby maybe persisting an invalid object state), simply tell AASM to skip the validations:

class Job < ActiveRecord::Base
  include AASM

  aasm :skip_validation_on_save => true do
    state :sleeping, :initial => true
    state :running

    event :run do
      transitions :from => :sleeping, :to => :running
    end

    event :sleep do
      transitions :from => :running, :to => :sleeping
    end
  end

end

Mongoid

AASM also supports persistence to Mongodb if you're using Mongoid. Make sure to include Mongoid::Document before you include AASM.

class Job
  include Mongoid::Document
  include AASM
  field :aasm_state
  aasm do
    ...
  end
end

Automatic Scopes

AASM will automatically create scope methods for each state in the model.

class Job < ActiveRecord::Base
  include AASM

  aasm do
    state :sleeping, :initial => true
    state :running
    state :cleaning
  end

  def sleeping
    "This method name is in already use"
  end
end

class JobsController < ApplicationController
  def index
    @running_jobs = jobs.running
    @recent_cleaning_jobs = jobs.cleaning.where('created_at >=  ?', 3.days.ago)

    # @sleeping_jobs = jobs.sleeping   #=> "This method name is in already use"
  end
end

If you don't need scopes (or simply don't want them), disable their creation when defining the AASM states, like this:

class Job < ActiveRecord::Base
  include AASM

  aasm :create_scopes => false do
    state :sleeping, :initial => true
    state :running
    state :cleaning
  end
end

Transaction support

Since version 3.0.13 AASM supports ActiveRecord transactions. So whenever a transition callback or the state update fails, all changes to any database record are rolled back. Mongodb does not support transactions.

If you want to make sure a depending action happens only after the transaction is committed, use the after_commit callback, like this:

class Job < ActiveRecord::Base
  include AASM

  aasm do
    state :sleeping, :initial => true
    state :running, :after_commit => :notify_about_running_job

    event :run do
      transitions :from => :sleeping, :to => :running
    end
  end

  def notify_about_running_job
    ...
  end
end

If you want to encapsulate state changes within an own transaction, the behavior of this nested transaction might be confusing. Take a look at ActiveRecord Nested Transactions if you want to know more about this. Nevertheless, AASM by default requires a new transaction transaction(:requires_new => true). You can override this behavior by changing the configuration

class Job < ActiveRecord::Base
  include AASM

  aasm :requires_new_transaction => false do
    ...
  end

  ...
end

which then leads to transaction(:requires_new => false), the Rails default.

Column name & migration

As a default AASM uses the column aasm_state to store the states. You can override this by defining your favorite column name, using :column like this:

class Job < ActiveRecord::Base
  include AASM

  aasm :column => 'my_state' do
    ...
  end

end

Whatever column name is used, make sure to add a migration to provide this column (of type string):

class AddJobState < ActiveRecord::Migration
  def self.up
    add_column :jobs, :aasm_state, :string
  end

  def self.down
    remove_column :jobs, :aasm_state
  end
end

Inspection

AASM supports a couple of methods to find out which states or events are provided or permissible.

Given the Job class from above:

job = Job.new

job.aasm.states
=> [:sleeping, :running, :cleaning]

job.aasm.states(:permissible => true)
=> [:running]
job.run
job.aasm.states(:permissible => true)
=> [:cleaning, :sleeping]

job.aasm.events
=> [:run, :clean, :sleep]

Installation

Manually from RubyGems.org

% gem install aasm

Or if you are using Bundler

# Gemfile
gem 'aasm'

Building your own gems

% rake build
% sudo gem install pkg/aasm-x.y.z.gem

Latest changes

Take a look at the CHANGELOG for details about recent changes to the current version.

Questions?

Feel free to

• create an issue on GitHub
• ask a question on StackOverflow (tag with aasm)
• send us a tweet @aasm

Maintainers

• Scott Barron (2006–2009, original author)
• Travis Tilley (2009–2011)
• Thorsten Böttger (since 2011)

Warranty

This software is provided "as is" and without any express or implied warranties, including, without limitation, the implied warranties of merchantibility and fitness for a particular purpose.

License

Copyright (c) 2006-2014 Scott Barron

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.