Changing state using before_transition/after_failure callbacks?

[Jan Hecking]

Is it possible to use event callbacks to trigger another transition to a different state?

E.g. I have an event authorize that transitions an object from the new to the authorized state. There is a before_transition callback defined for the event that queries some remote web service. If the query succeeds the transition to authorized should complete. But if it fails I would like to transition the object to the failed state instead.

Here is simplified example:

class StateTest

  attr_accessor :state

  state_machine :initial => :new, :use_transactions => false do

    state :new

    state :authorized

    state :failed

    event :authorize do

      transition :new => :authorized

    end

    event :error do

      transition any => :failed

    end

    around_transition :do => :log_transition

    before_transition :on => :authorize, :do => :handle_authorization

    after_failure :do => :handle_error

  end

  def handle_authorization(transition)

    result = false # authorize_with_some_remote_service()

    if !result

      throw :halt

    end

  end

  def handle_error(transition)

    obj = transition.object

    obj.error unless transition.to == :failed

  end

  def log_transition(transition)

    puts "starting transition #{transition.from} -> #{transition.to}"

    yield

    completed = true

    puts "completed transition #{transition.from} -> #{transition.to}"

  ensure

    puts "aborted transition #{transition.from} -> #{transition.to}" unless completed

  end

end

When executing the authorize event this is what the log_transition method outputs:

1.9.3p392 :287 > s = StateTest.new

 => #<StateTest:0x007fb18e8be3d0 @state="new">

1.9.3p392 :288 > s.authorize

starting transition new -> authorized

aborted transition new -> authorized

starting transition new -> failed

completed transition new -> failed

 => false

1.9.3p392 :289 > s

 => #<StateTest:0x007fb18e8be3d0 @state="new">

So even though the new -> failed transition completed the object is still in the new state afterwards. I tried this example with and without using transactions but it does not make a difference. I've tried various other variations as well, such as setting the state property directly instead of using the error event. But whatever I tried, at the end of the authorized event the state was reverted back to new.

Is it possible to automatically trigger another transition if a transition fails?

One way I could think of solving this is to change my event name from authorize to authorization_success and turn authorize into a regular instance method that handles the web service query and then either triggers the authorization_success or error events:

class StateTest2

  attr_accessor :state

  state_machine :initial => :new, :use_transactions => false do

    state :new

    state :authorized

    state :failed

    event :authorization_success do

      transition :new => :authorized

    end

    event :error do

      transition any => :failed

    end

    around_transition :do => :log_transition

  end

  def authorize

    result = false # authorize_with_some_remote_service()

    if result

      authorization_success

    else

      error

    end

    result

  end

  def log_transition(transition)

    puts "starting transition #{transition.from} -> #{transition.to}"

    yield

    completed = true

    puts "completed transition #{transition.from} -> #{transition.to}"

  ensure

    puts "aborted transition #{transition.from} -> #{transition.to}" unless completed

  end

end

This works as expected:

1.9.3p392 :001 > s = StateTest2.new

 => #<StateTest2:0x007f87e2335880 @state="new">

1.9.3p392 :002 > s.authorize

starting transition new -> failed

completed transition new -> failed

 => true

1.9.3p392 :003 > s

 => #<StateTest2:0x007f87e2335880 @state="failed">

However, the first approach has several nice properties that I would like to retain:

• I can call obj.can_authorize? to check whether a transition to the authorized state is possible. In the second approach I would have to call obj.can_authorization_success? which is not as nice or intuitive.
• I loose the automatic logging through the log_transition callback for the actual authorization process.

Any solutions?

Jan

[Greg Vaughn]

I'm fighting with the same sort of thing. I'm new to the gem and the list, but I don't see any replies. Was there a resolution to the problem posted somewhere else?

-Greg Vaughn

[Jan Hecking]

Hi Greg,

Sorry that I haven't responded to your private mail earlier - it's been a busy week here...

Anyway, the way I eventually decided to address this issue was by overriding the event method generated by state_machine - StateTest#authorize in my prev. example. In that method I handle some remote API calls first then then either call super() to transition to the authorized state of else I transition to a failed state instead. E.g. something like this:

class StateTest

  attr_accessor :state

  state_machine :initial => :new do

    state :new

    state :authorized

    state :failed

    event :authorize do

      transition :new => :authorized

    end

    event :fail do

      transition any => :failed

    end

  end

  def authorize

    result = false # authorize_with_some_remote_service()

    if result

      super()

    else

      fail()

    end

  end

end

I still would have preferred a solution using before and after call-backs to control the transitions as per my orig. example. It just feels more elegant and I don't need to override any automatically generated methods. But state_machine's transaction handling is preventing this. So this solution is working for us now even if not quite as nice.

Hope this helps!

Jan

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