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From [[Gogaruco 2012]]

Presenter: [[Erik Michaels-Ober]]


Erik Michaels-Ober is the creator of one of the most popular Rails engines: RailsAdmin. He is also the author of MultiXML and T, a command-line power-tool for Twitter. When he's not hacking on those projects, he helps maintain OmniAuth among other Ruby gems and open-source Rails applications, including He was a 2010 Ruby Summer of Code mentor and a 2011 Google Summer of Code mentor, while serving as a Code for America Fellow. He lives in San Francisco, California.


As of Rails 3.2, plugins are officially deprecated. They have been replaced by Rails Engines, a class of public methods for hooking into Rails applications. This session will explain how to write a Rails Engine from scratch or convert your existing plugin to an engine. It will also cover best practices for making your engine play well with others, and when to factor pieces of your existing Rails application code into an engine for reuse across multiple applications.


  • Rails Engines dates from Oct. 31, 2005!
  • DHH was initially against engines.
  • James Adam, creator of engines: "It would be madness to include them in the core of rails".
  • Merb Slices: "Little slices of MVC cake"
  • Erik wrote MerbAdmin.
  • Dec. 23, 2008 - Rails/Merb merger.
    • This killed Merb, but brought many ideas from Merb into Rails.
  • The Russian Doll Pattern - 2009 talk by Wycats and Carl Lerche
  • 2010: Ruby Summer of Code - Piotr Sarnacki - Mountable Engines
  • Old plugin style deprecated in rails 3.2, removed in 4.0

Memorable Quotes

Notes from @simeonwillbanks


All a rails app is a bootable rails engine
Engines do not know how to start themselves, but are everything else, MVC
Engines can plugin to engines

Rails engines are fundamental building block of rails


James Adam is creator of rails engines
1st commit to engines plugin was 10/31/2005
Rails 0.14.2
Really old concept/feature

DHH was against engines!

James Adam thought it would be "Madness" to have engines in rails

Merb slices introduced on 05/21/2008 Slice out bits of functionality and use across multiple applications

Rails and Merb merge on 12/23/2008
Concepts of merb, including engines, were built into rails core

The Russian Doll Pattern by Carl Lerche and Yehuda Katz
Apps within apps

Ruby Summer of Code 2010
Piotr Sarnacki used Russian Doll Pattern and implemented it in rails
Mentored by Carl, Yehuda and Jose Valim

Bogdan Gaza rebuilt MerbAdmin

Rails 3.0 uses engines!
Pain point was assets which had to be copied via rake task or generator

Rails 3.1 Asset Pipeline, FTW
Puts every engine in load path for assets
Looks in all engines to load assets
Engines became first class citizens!

Rails 3.2 deprecates vendor/plugins
The way forward is engines
Rails 4, engines are the only way

Write a Rails engine

Easy, same as writing a rails app
$ rails plugin new my_engine --mountable
very similar file system

  • Engines are namespaced by modules
  • Engines inherit from Rails::Engine which adds engine to load path
  • Engine is not added to Gemfile, specify via gemspec
  • Can have custom rake tasks in lib/tasks and scoped to engine name
  • Can define generators lib/generators and scoped by engine name
  • Can write migrations db/migrations and scoped by engine name
  • rake my_engine::install::migrations only copy in new migrations
  • Routes are defined in engine and engine is mounted in client app routes at a certain path

Testing a Rails engine

Best practice is

  1. to create a dummy app in test directory
  2. mount engine in dummy app
  3. test the dummy app

For more info:

When should I write an engine?

  1. If you are at a point and you have two apps which need to share logic, slice out the logic into a gem to be shared.
  2. A base like scaffolding, assets or template which can be mounted into multiple apps and reused