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Metasploit::Concern allows you to define concerns in app/concerns that will automatically be included in matching classes. It can be used to automate adding new associations to ActiveRecord::Base models from gems and Rails::Engines.


Metasploit::Concern is versioned using semantic versioning 2.0. Each branch should set Metasploit::Concern::Version::PRERELEASE to the branch SUMMARY, while master should have no PRERELEASE and the PRERELEASE section of Metasploit::Concern::VERSION does not exist.


Add this line to your application's Gemfile:

gem 'metasploit-concern'

And then execute:

$ bundle

This gem's Rails::Engine is not required automatically. You'll need to also add the following to your config/application.rb:

require 'metasploit/concern/engine'

Or install it yourself as:

$ gem install metasploit-concern

Supporting concerns

Metasploit::Concern support is a cooperative effort that involves the classes from the gem being setup to allow downstream dependents to inject concerns.

In order for Metasploit::Concern to load concerns for app/concerns, the class on which Module#include will be called must support ActiveSupport load hooks with a specific name format. You can run the appropriate load hooks at the bottom of the class body:

class GemNamespace::GemClass < ActiveRecord::Base
  # class body


Include the shared examples from Metasploit::Concern in your spec_helper.rb:

Dir[Metasploit::Concern.root.join("spec/support/**/*.rb")].each do |f|
  require f

To verify that your classes call correctly, you can use the '' shared example:

# spec/app/models/gem_namespace/gem_class_spec.rb
describe GemNamespace::GemClass do
  it_should_behave_like ''

Using concerns

Concerns are added in downstream dependents of gems that support concerns. These dependents can be a Rails::Engines or full Rails::Application.



Add this line to your application's config/application.rb:

config.paths.add 'app/concerns', autoload: true

Or if you're already using config.autoload_paths +=:

config.autoload_paths += config.root.join('app', 'concerns')


Add this line to your engine's class body:

module EngineNamespace
  class Engine < ::Rails::Engine
    config.paths.add 'app/concerns', autoload: true


Define concerns for class under app/concerns by creating files under directories named after the namespaced class names:

$ mkdir -p app/concerns/gem_namespace/gem_class
$ edit app/concerns/gem_namespace/gem_class/my_concern.rb

Inside each concern, make sure the module name matches file name:

module GemNamespace::GemClass::MyConcern

Each concern is included using Module#include which means that the included method on each concern will be called. Using ActiveSupport::Concern allow you to add new associations and or validations to ActiveRecord::Base subclass:

module GemNamespace::GemClass::MyConcern
  extend ActiveSupport::Concern

  included do
    # Associations

    # @!attribute widgets
    #  Widgets for this gem_class.
    #  @return [ActiveRecord::Relation<Widget>]
    has_many :widgets,
             class_name: 'Widget',
             dependent: :destroy,
             inverse_of :gem_namespace_gem_class


Metasploit::Concern::Engine defines the 'metasploit_concern.load_concerns' initializer, which sets up ActiveSupport.on_load callbacks. If you depend on a feature from a concern in your initializers, it is best to have the initializer declare that it needs to be run after 'metasploit_concern.load_concerns:

initializer 'application_or_engine_namespace.depends_on_concerns', after: 'metasploit_concern.load_concerns' do
  if GemNamespace::GemClass.primary.widgets.empty?'No Widgets on the primary GemClass!')