This document discusses Rails engines and gems. It explains that engines allow extracting common functionality into reusable modules. Engines use the Rails::Engine class and Railtie to integrate with Rails applications. The document also covers creating gems, testing engines, mounting engines to host apps, and overriding engine classes and views.

2. Problem
 monolithic application
 no reusability
 slow tests
 bigger application, more mess

3. Possible solutions
 extract common functionality to modules
 create gems
 tie gem with rails application with railtie
 rails engine

4. What is a Gem ?
 packaged application or library
 code
 tests
 documentation
 gemspec

5. Creating gem
 bundle gem simplify ├── Gemfile
├── LICENSE.txt
├── README.md
 structure ├── Rakefile
 code (lib/) ├── lib
│ ├── simplify
 tests (test/ or spec/) │ │ └── version.rb
 documentation (doc/ README) │ └── simplify.rb
└── simplify.gemspec
 executable files (bin/)
 information about gem
 simplify.gemspec

6. Releasing Gem
 change version
 run `bundle`
 commit changes
 run `rake release`
$ rake release
stupid_spam_protection 0.0.2 built to pkg/stupid_spam_protection-0.0.2.gem
Tagged v0.0.2
Pushed git commits and tags
Pushed stupid_spam_protection 0.0.2 to rubygems.org

7. in-memory testing with sqlite
spec/spec_helper.rb
require "active_record"
require "sqlite3”
ActiveRecord::Base.establish_connectio
n{
:adapter => "sqlite3”,
:database => ":memory:”
}
load "db/schema.rb" db/schema.rb
ActiveRecord::Schema.define do
RSpec.configure do |config| create_table "items", :force => true do |t|
config.around do |example| t.string "name”
ActiveRecord::Base.transaction do t.datetime "created_at", :null => false
example.run t.datetime "updated_at", :null => false
raise ActiveRecord::Rollback end
end
end
end

8. testing with mysql
database
spec/spec_helper.rb
begin
ActiveRecord::Base.establish_connection(config)
rescue
ActiveRecord::Base.establish_connection(config.merge('database' => nil))
ActiveRecord::Base.connection.create_database(config['database'], {
:charset => 'utf8‟,
:collation => 'utf8_unicode_ci‟})
end config = HashWithIndifferentAccess.new({
:adapter => "mysql2",
load "db/schema.rb" :host => "127.0.0.1",
:username => "root",
RSpec.configure do |config| :password => "",
# the same from previous slide :database => ”database_name”
end })

9. Tie gem to Rails app
Rails::Railtie
 extend Rails framework
 load all needed dependencies
 several hooks methods for all needs
require "wnm_support/railtie" if defined?(Rails) lib/wnm_support.rb
module WnmSupport lib/wnm_support/railtie.rb
class Railtie < Rails::Railtie
initializer "wnm_support.view_helpers" do
ActionView::Base.send :include,
BoolToHuman
end
end
end

10. Railtie hooks 1
 Plugin hooks
 initializer
 generators
 rake_tasks
 console
initializer "wnm_support.active_record_ext" do
ActiveSupport.on_load(:active_record) do
ActiveRecord::Base.send :include,
WnmSupport::ActiveRecordExt::DestroyValidation
end
end

11. Railtie hooks 2
 Rails configuration hooks
 to_prepare (once in production, every request in development)
 before_initialize
 after_initialize
 app_middleware
 before_configuration
 before_eager_load
 generators
config.to_prepare do
AppController.send(:include,
Controller::Authentication)
end

12. What is Rails Engine ?
 every rails app is an engine
 Rails::Engine < Rails::Railtie
 all together
 gem skeleton
 railtie
 structure for app
 dummy app for testing
 assets

13. Engine walkthrough
 generate new engine
 basic commands
 testing
 mounting to host app
 overriding defaults

14. Generating Engine
 rails plugin new blorgh --mountable –
full
 everything (assets, controller, models,
views) are inside Blorgh namespace
module Blorgh
class Engine < ::Rails::Engine
isolate_namespace Blorgh
end
end

15. Basics commands
 in root directory of the engine
 bundle
 rake
 rspec
 rails generate # generate code for engine
 in spec/dummy or test/dummy location
 rails console
 rails server
 rails generate # generate code for dummy app

16. Testing
 generally as in standard rails application
 engine is tested inside dummy app
 small problems with factory girl
 explicite model class in factory girl
 include routes for request tests
FactoryGirl.define do
factory :user, :class => Blorgh ::User do
sequence(:login) { |n| "admin_#{n}" } RSpec.configure do |config|
password "too_secret" config.include Blorgh ::Engine.routes.url_helper
password_confirmation { password } end
end
end

17. Mounting Engine to host
application
 Gemfile
 gem “blorgh”, :path => “~/Sites/blorgh”
 install migration
 `rake blorgh:install:migrations`
 load seeds data
 Blorgh::Engine.load_seed # in rails console
 require assets if needed
 *= require blorgh/application # assets/stylesheets/application.css
 mount to routes.rb
 mount Blorgh::Engine => "/blorgh"

18. Get to engine and get
back
 routes to engine from host app
 prefix with engine name
 example engine “blorgh”
 <%= link_to “Users”, blorgh.users_path %>
 routes from engine to host app
 prefix with main_app
 example engine “blorgh”
 <%= link_to “Home page”,main_app.root_path %>

19. Overriding engine classes
 Controller, Model, etc.
 reopening classes
 class_eval, module_eval
module MySite
class Railtie < ::Rails::Railtie
config.to_prepare do
Dir["app/decorators/**/*.rb"].each do |path|
load path app/decorators/models/blorgh/article.rb
end
end Blorgh::Article.class_eval do
end has_many :users, :class_name => „User'
end end

20. Overriding engine views
 simple copy and paste view file that you want to
override to the same location in host app

21. Application Modules with
Rails Engine
 every new engine means new namespace
 application modules means for us more engines with
same namespace
 inspiration from refinerycms

22. The same namespace
different engines
module Wnm module Wnm
module Core module Pages
class Engine < ::Rails::Engine class Engine < ::Rails::Engine
isolate_namespace Wnm isolate_namespace Wnm
engine_name :wnm_core engine_name :wnm_pages
end end
end end
rake wnm_core:install:migrations rake wnm_pages:install:migrations
Wnm::Core::Engine.load_seed Wnm::Pages::Engine.load_seed
both are nested in `Wnm` namespace

23. Developing from local
Production from git
 bundler does not allow to have same package from
different sources in Gemfile
export LOAD_GEMS_FROM_LOCAL=1 .rvmrc
if ENV["LOAD_GEMS_FROM_LOCAL"] == "1" Gemfile
gem "wnm_core", :path => "~/Sites/wnm_core"
gem "wnm_pages", :path => "~/Sites/wnm_pages"
gem "wnm_customers", :path => "~/Sites/wnm_customers"
else
gem "wnm_core", :git => "git@…/wnm_core.git", :tag => "v1.0.0"
gem "wnm_pages", :git => "git@…/wnm_pages.git", :tag => "v1.0.0"
gem "wnm_customers", :git => "git@.../wnm_customers.git", :tag => "v1.0.0"
end

24. Deploying
 change LOAD_GEMS_FROM_LOCAL to 0 and run
`bundle`
 commit and push
 `cap deploy`

25. Tips
 if you are overriding views heavily always prefix routes
path with engine module name
<%= link_to page.name, wnm_page.page_path(page), :title => page.name %>
 use class methods instead of constants
 always write full path to class/module if it is in
namespace
 ::ApplicationController instead of ApplicationController
 testing is essential

26. Why use Rails::Engine ?
 reusable code
 code
 hole application
 assets
 modular thinking
 gem with MVC
 faster test
 testing gem in dummy app infrastructure
 some well known engines
 devise, kaminari, refinerycms

27. Problems
 if you are bending engines weird thinks can happen
 a lot of weird errors
 application does not run in development mode from git
sources
 tests passed, but in browser it is not running
 in browser it is running but tests are throwing exceptions
 overriding classes in host app
 helper methods from host app does not work in overridden
views
 some things from documentation does not work at all

28. Why we did this like that ?
 a lot of our project are in general the same
 build core for site quickly
 “scaffold” for application
 rails engine is infrastructure for modular application
 we can have backend and frontend in the same
application module

29. Sources
 http://guides.rubygems.org/what-is-a-gem/
 http://www.engineyard.com/blog/2010/extending-rails-3-with-railties/
 http://www.slideshare.net/AndyMaleh/rails-engine-patterns
 http://edgeguides.rubyonrails.org/engines.html
 http://edgeapi.rubyonrails.org/classes/Rails/Railtie.html
 http://edgeapi.rubyonrails.org/classes/Rails/Engine.html
 http://pivotallabs.com/users/shagemann/blog/articles/1994-migrating-
from-a-single-rails-app-to-a-suite-of-rails-engines
 http://railscasts.com/episodes/301-extracting-a-ruby-gem
 http://railscasts.com/episodes/303-publishing-a-gem
 http://railscasts.com/episodes/277-mountable-engines