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Rule

What is it?

Rule is a state machine / rule engine. It does things.

How do I get started?

  1. Add it to your gemfile.

    gem "rule", :git => "https://github.com/innovatis/rule"
    
  2. Add a “state” column and this code to your model:

    rule_engine MyAwesomeRuleEngine
    
  3. create a file at app/rule_engines/my_awesome_rule_engine.rb

    class MyAwesomeRuleEngine < Rule::Engine::Base
    
      state :new
      state :ongoing
      state :closed
    
      initial_state :new
    
      terminal_state :closed
    
      transition :new, :ongoing
        validate IsInProgressRule
      end
    
      transition :ongoing, :closed
        assert_presence_of object.closed_at, "Closed At"
      end
    
    end
  4. Create a file at app/rules/is_in_progress_rule.rb

    class IsInProgressRule < Rule::Base
      def validate
        @object.in_progress.present?
      end
    end
    
  5. Set your application up to run the rules.

    # wherever it makes sense...
    my_object.run_rules # advances state as far as possible and automatically saves
    

Explain all that stuff.

Okay.

The Rule::Engine::Base subclass defines states and transitions. Each transition block contains a number of assertions and an optional priority. If any of the assertions does not pass, the state will not follow this transition. The priority decides which transition to use in the case of two possible transitions out of the current state both being valid. Use it by specifying “priority :high” or “priority :low” in the transition block.

It often makes sense to bundle a group of assertions into a Rule class. These are defined in app/rules, and have a ‘validate` method containing assertions. In the future, rules will also have callbacks that fire when the state is advanced through a transition including that rule.

If this didn’t answer your questions, throw things at Burke until he explains.

Copyright © 2011 Burke Libbey / Innovatis. MIT License.