State Machines

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State machines represent a very big and complex topic in the world of computer science; navigating to the state machine gem's ReadMe file can make grown men cry. But state machines don't have to the be that complicated!

Take a simple example of moderating a comment for a blog post. The user is able to flag an inappropriate comment. That causes the comment to appear in a review section for the site admin. The site admin then decides whether he or she wishes to approve or remove the comment. Only approved comments are shown.

First, install the state machine gem:

gem 'state_machine'

bundle

Don't forget to add state to your model (string).

Then edit your model to include states:

state_machine :initial => :approved do
  state :approved
  state :flagged
  state :removed

  event :flag do
    transition :approved => :flagged
  end

  event :approve do
    transition all => :approved
  end

  event :remove do
    transition :flagged => :removed
  end
end

Using the all transition means that you can approve a comment no matter what state your record is.

Congratulations, you just created a state machine! Now you could perform actions such as:

comment.approve
comment.flag
comment.remove