#392 A Tour of State Machines pro

Nov 17, 2012 | 12 minutes | Active Record, Testing, Plugins

Here I show how three popular state machine gems can be used to clean up a list of boolean/datetime fields. I also show a custom solution which keeps track of the history of events.

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Mark W Windholtz about 12 years ago

Thanks Ryan. Good review.

What bothers me about most of these is that they keep state behavior all in one class.
That can make for a big state model class.

An alternative is something like StateObjects
which uses the State Design Pattern from the Gang of Four book. The state behavior is moved to small separate classes. StateObjects has less support for transitions/events but more for extracting the behavior.

Benjamin Harmless Huber about 12 years ago

http://blog.envylabs.com/post/29045466981/the-rails-state-machine

probably this would be nice too in the show notes.

it seems to me that not very much people know about the built-in version.

just used it once. i dont even know if its also built-in in the current version of rails

Benjamin Harmless Huber about 12 years ago

ok seems it got lost in 3.x ;) sorry for bothering

Balazs Nagy about 12 years ago

It has been extracted to transitions gem, even before rails 3 came out.

Robert L. Carpenter about 12 years ago

Is the Atom feed broken, or am I missing something?

I haven't received any updates in google Reader since Turbolinks. Looking at the XML at http://feeds.feedburner.com/railscasts, it hasn't been updated since then either.

caleb bron about 12 years ago

The feeds only show the free ones. Everything since Turbolinks has been revised or pro.

Andy Waite almost 12 years ago

The RSS at the top right of the page will give you a personalised link containing the pro episodes.

http://railscasts.com/subscriptions/...../episodes.rss

paolotax about 12 years ago

Any idea on how to implement with postgresql

          ruby
        
def self.with_last_state(state)
  order("id desc").group("order_id").having(state: state)
end

and the relative scope in Order model

ehoffmann almost 12 years ago

This worked for me:

          ruby
        
class Order 
...
def self.open_orders
    joins(:events).where("order_events.id IN (SELECT MAX(id) FROM order_events GROUP BY order_id) AND state = 'open'")
end

Eric Boehs over 7 years ago

Building on your response, I replaced Ryan's with_last_state with this MAX(id) solution:

          ruby
        
class OrderEvent
  def self.with_last_state state
    where state: state, id: select('MAX(id)').group(:order_id)
  end
end

Shane Pinnell almost 12 years ago

Any luck with this?

Shane Pinnell almost 12 years ago

I just removed the grouping and it works fine, not sure what purpose the group("order_id") was serving, maybe I am missing something?

regedarek about 12 years ago

Guys, do you know how to after updating object always change state to awaiting_review? I am using workflow gem.

Daniel Libanori almost 12 years ago

Unlike AASM and Workflow, state_machine supports multiple state machines at same object/class.

Shane Pinnell almost 12 years ago

Implementing the roll your own method of this for a project of mine and I keep getting an error when I attempt to find all "draft" or "open" or "whatever" orders:

ERROR: column "orders.id" must appear in the GROUP BY clause or be used in an aggregate function
LINE 1: SELECT "orders".* FROM "orders" INNER JOIN "order_events" ON...

Any ideas?

Ian Drysdale over 11 years ago

Yeah - I'm experiencing this too. I tweaked the code but now it appears to finds all the orders that are both in that state currently, or have at some point been in that particular state, which isn't quite working either.

Mohamad El-Husseini over 11 years ago

Did you solve this issue?

Ian Drysdale over 11 years ago

I ended up using:

          ruby
        
def self.open
  joins(:events).where("user_events.id IN (SELECT MAX(id) FROM user_events GROUP BY user_id) AND state = 'open'")
end

Hope that helps.

Mohamad El-Husseini over 11 years ago

Were you able to solve this?

Aaron Cruz almost 12 years ago

I use the old hash syntax for state transitions for the same reason. An good example of where language syntax flexibility is neat.

Jonathan Maddison almost 12 years ago

Wow, this is great.

I was spending so much time trying to get my head around that exact problem (tracking order status). I felt like it was getting way more complicated than it should be.

Never realised state machines were so practical.

Jonathan Camenisch over 11 years ago

For an introduction to state machines, I would highly recommend this series of lectures: http://www.youtube.com/watch?v=HyUK5RAJg1c

It's fun to watch, and he goes through enough examples to make the concept very familiar by the time you get through it.

phillyslick over 11 years ago

Rafael Barbolo about 11 years ago

onvard about 10 years ago

Is there any way to create custom workflows for each user? So a user may design his own workflow, and an object goes through the states.

Andy L about 10 years ago

I need the same thing!

Aaron Darcy about 8 years ago

Information could be stolen that may really hurt your business, so make sure that something that is amazingly sensitive is securely guaranteed in locked file cabinets which all computers are password protected.

AzzpAdmin over 7 years ago

Great information thanks!