(Note! Module#concerning is included in Rails 4.1. You can still use this library, but it will defer to Active Support's implementation if available.)

We often find ourselves with a medium-sized chunk of behavior that we'd like to extract, but only mix in to a single class.

Extracting a plain old Ruby object to encapsulate it and collaborate or delegate to the original object is often a good choice, but when there's no additional state to encapsulate or we're making DSL-style declarations about the parent class, introducing new collaborators can obfuscate rather than simplify.

The typical route is to just dump everything in a monolithic class, perhaps with a comment, as a least-bad alternative. Using modules in separate files means tedious sifting to get a big-picture view.

class Todo
  # Other todo implementation
  # ...

  ## Event tracking
  has_many :events

  before_create :track_creation
  after_destroy :track_deletion

  def self.next_by_event
    # ...
  end


  private
    def track_creation
      # ...
    end
end

Noisy syntax.

class Todo
  # Other todo implementation
  # ...

  module EventTracking
    extend ActiveSupport::Concern

    included do
      has_many :events
      before_create :track_creation
      after_destroy :track_deletion
    end

    module ClassMethods
      def next_by_event
        # ...
      end
    end

    private
      def track_creation
        # ...
      end
  end
  include EventTracking
end

Once our chunk of behavior starts pushing the scroll-to-understand it boundary, we give in and move it to a separate file. At this size, the overhead feels in good proportion to the size of our extraction, despite diluting our at-a-glance sense of how things really work.

class Todo
  # Other todo implementation
  # ...

  include TodoEventTracking
end

By quieting the mix-in noise, we arrive at a natural, low-ceremony way to separate bite-sized concerns.

  class Todo
    # Other todo implementation
    # ...

    concerning :EventTracking do
      included do
        has_many :events
        before_create :track_creation
        after_destroy :track_deletion
      end

      class_methods do
        def next_by_event
          # ...
        end
      end

      private
        def track_creation
          # ...
        end
    end
  end

  Todo.ancestors
  # => Todo, Todo::EventTracking, Object

This small step has some wonderful ripple effects. We can

• grok the behavior of our class in one glance,
• clean up monolithic junk-drawer classes by separating their concerns, and
• stop leaning on protected/private for crude "this is internal stuff" modularity.