Extending controllers of a Rails 3 Engine in the main app

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By design, classes in a Rails::Engine are supposed to be scoped to the engine. That way they don’t introduce strange bugs by accidentally stomping all over code loaded in the main app or by other engines. Monkeypatching ActiveSupport::Dependencies to mix engines across-the-board is a really bad workaround.

Just use a Rails::Railtie, instead. They have all the same functionality, but aren’t scoped the same way as an engine. You have access to the entire rails app stack (including engines). It’s a more surgical approach. 

module MyModule

  module SomeModelExtensions
    # Called when this module is included on the given class.
    def self.included(base)
      base.send(:include, InstanceMethods)
      base.extend(ClassMethods)
    end

    module ClassMethods
      def some_new_class_method
        # do stuff...
      end
    end

    module InstanceMethods
      def some_new_instance_method
        # do stuff...
      end
    end

  end

  module SomeControllerExtensions
    def self.included(base)
      base.send(:include, InstanceMethods)
      base.alias_method_chain :new, :my_module
    end

    module InstanceMethods
      # override the 'new' method
      def new_with_my_module
        # do stuff
      end
    end
  end

  class Railtie < ::Rails::Railtie

    # The block you pass to this method will run for every request in
    # development mode, but only once in production.
    config.to_prepare do
      SomeModel.send(:include, MyModule::SomeModelExtensions)
      SomeController.send(:include, MyModule::SomeControllerExtensions)
    end

  end

end

As far as file layout, railties look exactly like engines.

Further reading: Extending Rails 3 with Railties

And if you’re still confused, take a look at this git project which has a full implementation: https://github.com/jamezilla/bcms_pubcookie

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