using ActiveSupport::Concerns to extend Rails

rails_engines.md

Gist to describe the process of adding a namespaced isolated Rails Engine
to a typical Rails application and adding methods to one of the Rails Engine's
controller and model.

Index

1. Rails applicaiton all by itself.
2. Rails application with the Rails Engine (FooBar) mounted.
3. Rails applicaiton partially extending a controller/model from the Rails Engine.

1. Rails application all by itself

MyApp
 |--- controllers
 |             |--- a_controller.rb
 |             |--- b_controller.rb
 |             |--- c_controller.rb
 |
 |-------- models
 |             |--- a.rb
 |             |--- b.rb
 |             |--- c.rb
 |
 |-------- ... and other rails folders

2. Rails applicaiton with the Rails Engine (FooBar) mounted

MyApp
 |--- controllers
 |             |--- a_controller.rb
 |             |--- b_controller.rb
 |             |--- c_controller.rb
 |
 |-------- models
 |             |--- a.rb
 |             |--- b.rb
 |             |--- c.rb             
 |
 |---- config
 |       |---- application.rb ( require 'foobar' )
 |       |---- routes.rb ( mount FooBar::Engine. :at => '/foobar' )
 |
 |-------- Gemfile ... gem 'foo_bar'
 |
 |-------- ... and other rails folders


FooBar
    |--- app
    |     |--- controllers
    |     |           |--- one_controller.rb ( include FooBar::Concerns::Controllers::OneController )
    |     |           |--- two_controller.rb ( include FooBar::Concerns::Controllers::TwoController )
    |     |
    |     |-------- models
    |     |           |--- one.rb ( include FooBar::Concerns::Models::One )
    |     |           |--- two.rb ( include FooBar::Concerns::Models::Two )
    |     |--- views
    |             |--- one
    |             |     |--- add.html.erb
    |             |     |--- subtract.html.erb
    |             |     |--- multiply.html.erb
    |             |     |--- divide.html.erb
    |             |
    |             |--- two
    |                   |--- add.html.erb
    |                   |--- subtract.html.erb
    |                   |--- multiply.html.erb
    |                   |--- divide.html.erb    
    |             
    |---- lib
           |--- foo_bar.rb
           |--- foo_bar
                     |--- engine.rb
                     |--- cocnerns
                             |--- controllers
                             |           |--- one_controller.rb ( within file, extend ActiveSupport::Concern )
                             |           |--- two_controller.rb ( within file, extend ActiveSupport::Concern )
                             | 
                             |-------- models
                                         |--- one.rb ( within file, extend ActiveSupport::Concern )
                                         |--- two.rb ( within file, extend ActiveSupport::Concern ) 

3. Rails application partially extending a controller/model from the Rails Engine.

We want to add a rails engine (FooBar) and expand the functionality of the

• FooBar::Concerns::Controllers::OnesController in a_controller.rb
• FooBar::Concerns::Models::One in a.rb

MyApp
 |--- controllers
 |             |--- a_controller.rb ( include FooBar::Concerns::Controllers::OnesController )
 |             |--- b_controller.rb
 |             |--- c_controller.rb
 |
 |-------- models
 |             |--- a.rb ( include FooBar::Concerns::Models::One )
 |             |--- b.rb
 |             |--- c.rb             
 |
 |---- config
 |       |---- application.rb ( require 'foobar' )
 |       |---- routes.rb ( mount FooBar::Engine. :at => '/foobar' )
 |
 |-------- Gemfile ... gem 'foo_bar'
 |
 |-------- ... and other rails folders


FooBar
    |--- app
    |     |--- controllers
    |     |           |--- one_controller.rb ( include FooBar::Concerns::Controllers::OneController )
    |     |           |--- two_controller.rb ( include FooBar::Concerns::Controllers::TwoController )
    |     |
    |     |-------- models
    |     |           |--- one.rb ( include FooBar::Concerns::Models::One )
    |     |           |--- two.rb ( include FooBar::Concerns;:Models::Two )
    |     |--- views
    |             |--- one
    |             |     |--- add.html.erb
    |             |     |--- subtract.html.erb
    |             |     |--- multiply.html.erb
    |             |     |--- divide.html.erb
    |             |
    |             |--- two
    |                   |--- add.html.erb
    |                   |--- subtract.html.erb
    |                   |--- multiply.html.erb
    |                   |--- divide.html.erb    
    |             
    |---- lib
           |--- foo_bar.rb
           |--- foo_bar
                     |--- engine.rb
                     |--- concerns
                             |--- controllers
                             |           |--- one_controller.rb ( within file, extend ActiveSupport::Concern )
                             |           |--- two_controller.rb ( within file, extend ActiveSupport::Concern )
                             | 
                             |-------- models
                                         |--- one.rb ( within file, extend ActiveSupport::Concern )
                                         |--- two.rb ( within file, extend ActiveSupport::Concern ) 

I just followed this instruction, but I got: uninitialized constant Concerns::Models exception