Rails ActiveSupport Concerns
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Rails
Last update
2017-04-11
2017
04-11
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Put this template in your /app/(models|controllers)/concerns folder:

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# /app/(models|controllers)/concerns/my_module.rb
module MyModule
  extend ActiveSupport::Concern

  included do
    scope :disabled, -> { where(disabled: true) }
    before_save :my_instance method
    ...
  end

  # use this syntax with Rails 5
  class_methods do
    def my_class_method
      ...
    end
  end # class_methods

  # use this syntax with Rails 4
  module ClassMethods
    def my_class_method
      ...
    end
  end # module ClassMethods

  def my_instance_method
    ...
  end
end

Then load it in your model/controller:

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class MyModel < ActiveRecord::Base
  include MyModule
end

class MyController < ApplicationController
  include MyModule
end

NOTE: for Rails < 4 remember to configure autoload_paths in /config/application.rb like this:

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module MyAppName 
  class Application < Rails::Application
    # Custom directories with classes and modules you want to be autoloadable.
    # config.autoload_paths += %W(#{config.root}/extras)
    config.autoload_paths << Rails.root.join('app', 'models'     , "concerns")
    config.autoload_paths << Rails.root.join('app', 'controllers', "concerns")
  end
end

WARNING: Moldel and controller concerns have the same scope! If you define two files with the same name but different path (eg: /app/models/my_module.rb and /app/controllers/my_module.rb) they will clash and only the last one loaded will be used.

Sources: DHH, Rails Api, StackOverflow