#277 Mountable Engines

I have always been a big fan on Engines and use them religiously. All the love Engines are being shown in Rails 3.1 is getting me very excited for the final release.

Thank you Ryan!

Is there a memory consumption overhead when bundling a plugin as an engine compared to, say, a gem? Has anyone benchmarked this?

Great tutorial, as always! Anybody knows of any good engines that can be mounted inside an rails app to handle the common stuff such as marketing pages / helpdesk?

Engines look promising in Rails 3.1!

Wondering why you didn't use something like

rescue_from Exception, :with => :record_error

in the application controller. Is it so that the error is raised like it normally is?
In that case you could reraise it in record_error I think.

So, are these engines intended to be used as gems or they will live in the plugins directory?

Thanks Ryan for this screencast!

If people are interested to see what a bigger Rails 3.1 engine looks like, a few friends and I have built a forum system called forem: http://github.com/radar/forem. We would be happy to answer any questions anyone has about building an engine.

Interesting. I wonder why Rails is able to load assets and initializers and such, but not able to pick up the migrations... one would think it could just append the migration path to the list of places to search for and run from.

Quick question.

Let's say that the author of the main app that uses the engine, decides to mount it under a different path.
For instance:

mount Uhoh::Engine => "/failures", :as => "uhoh"

How would this influence the assets used by the engine.
I imagine that the image at the top of the Uhoh failures page would fail, as the engine is no longer in the /uhoh path?

<%= image_tag "uhoh/alert.png" %>

Or is this handled automatically by Rails?

If you're planning to build gems from your engines, it may be worthwhile to start with the latest from 3-1-stable instead of RC5. The generated gemspec file is now much more complete, including details such as author, email and homepage. Also, it now contains dependencies which are included in the Gemfile via the gemspec command (similar to the "bundle gem" approach).

Hi Ryan - nice 'Casts!

A suggestion/request for Railscasts: Any way you can add a link in each episode's "show notes" page to that episode's git source code page? That would make what is already an awesome resource even better!

Wow soooo awesome i always neglected engines but from now ill see how i can apply them to my apps

Isn't that should be "Episode 149: Rails Engines" instead of "Episode 249: Notifications in Rails 3" in show notes ??

Bravo!! Always been a big fan of Railscasts!
I'm wondering if there's an easy way to mount an engine into a subdomain, say, http://uhoh.example.com in this case?

What if i have a main app that would like say multiple blogs provided through some mountable blog engine. Do I then have to mount the blog engine each time for each blog and with a different path each time?

mount Blogifier::Engine => '/blog'
mount Blogifier::Engine => '/blog2'

Ryan, thanks for a great episode. I am curious, too, about the ins and outs of mounting the engine multiple times. I am not sure what the use case would be, but the question is an interesting one.

Mountable engines was totally new to me before I watched this screencast. Therefor I have a couple of questions..

Would a use case example be to create a authentication and profile engine, then a blog engine, a forum engine etc and combine them all together under a "umbrella" app?

Is the mentioned use case a good way to create better scalable apps?

I guess you can create a engine within a engine as well?

Good screencast. Can you please explain why we need engine? Can I keep creating apps without engine or do I have to do it with?

How do I test mountable engine with RSpec? If I follow instructions from episode #275, generators use Test:Unit instead of RSpec, even though I have rails-rspec gem installed.

I believe Rails 3.2.0.beta fixes the bug where it creates both app/views/layouts/application.html.erb and app/views/layouts/uhoh/application.html.erb (where "uhoh" represents any mountable engine name).

I was just playing around with this and noticed that the url helper prefixing seems to have changed. I am on Rails 3.1 rc5, but had to downgrade to Arel 2.1.4 to get things launched (which wouldn't seem to make a difference w/r/t this)

# undefined variable uhoh_engine
link_to  'Engine home', uhoh_engine.root_url

#works
link_to 'Engine home', uhoh.root_url

It seems that configuration you place inside lib/uhoh/engine.rb will affect your main app's configuration at test/dummy/config/application.rb

I did this in my engine:

config.generators.do |g|
   g.template_engine :haml
end

And now the main app uses haml for scaffold generation. That seems pretty counterintuitive, since it is within the class of the engine; think that's a bug I should file on Rails?

Thank you for the great cast! Is there a way to share the layout between main app and engine app?

pado

I wonder how can I access the assets data in a test?

I tried Dummy::Application.config.assets['application.js'] but it always returns nil for all the assets.

Probably I should initialise it somehow...

Using rails 3.1 for creating and running the engine gives me the following error.
"couldn't find file 'jquery'"
I have jquery-rails as dependency in the gemspec file and the gem is installed.

I get the same error for both the engine and the dummy app.
I've asked in both ruby-forum and on IRC but no one seems to know anything about it.

http://www.ruby-forum.com/topic/2484569

Would be great if someone could shed some light on this matter :)

I hate to be the negative one here, but this feature seems uncharacteristically over-engineered.

For example, it appears to require much more work to setup a Rails engine than before. You used to be able to simply add a plugin that included a config/routes.rb file, and those routes would automagically be mounted in your main application. Is that still the case? Is there any reason to go through all these steps if you're not making a shared gem?

Last point: the syntax of
mount Uhoh::Engine => "/failures"
seems backwards to me. I would have expected:

mount "/failures" => Uhoh::Engine
instead, which seems a lot more intuitive and consistent.

Rails 3.1.1, 3.1.0: problem with routing. No route matches:

Started GET "/uhoh/" for 127.0.0.1 at 2011-10-17 16:46:27 +0400

ActionController::RoutingError (No route matches [GET] "/uhoh"):
  

Rendered /home/r_wilco/.rvm/gems/ruby-1.9.2-p180@rails-3.1.0/gems/actionpack-3.1.0/lib/action_dispatch/middleware/templates/rescues/routing_error.erb within rescues/layout (0.8ms)

Tried it with Rails 3.1.1.rc3 but got
uninitialized constant Uhoh (NameError) when trying
to mount the engine from another rails application.

Then I thought perhaps I have to do build/install the plugin
as a gem. After doing this and including the gem uhoh in the
main app's Gemfile the problem went away.
Not sure if this is correct.

Can any one look at the following question related to mountable engines? I urgently need it to solve,

http://stackoverflow.com/questions/9310287/rails-3-1-engines-gemspec-and-config-initializers

in rails 3.2.3 you need to run your dummy server with the

rails s

command from inside the tests/dummy folder, not at the root of the engine

docs:

http://edgeguides.rubyonrails.org/engines.html

Hi,

thanks for this great cast. I wrote a small engine with two controllers and now I want to integrate this controllers into the devise based security infrastructure. In my app controllers I'm using a before_filter :backend_rights_required. How can I add this filter to my engine controllers as well from the main app without changing the engine as self?

Thanks a lot
Dirk

Does anyone know if Mongoid can be used instead of activerecord?

It seems that when using the HttpHelpers in an Engine's routes file, I keep getting "uninitialized constant Controller". But, if I use a URL matcher in the Engine's routes file, it routes correctly.

For instance in the dummy application I have config/routes.rb:

Rails.application.routes.draw do
  mount MyEngine::Engine => "/myengine"
end

In the engine, if the config/routes.rb look like:

MyEngine::Engine.routes.draw do
  get "things/index"
end

... then, rake routes in the dummy application looks like:

$ rake routes
Routes for MyEngine::Engine:
  things_index GET /things/index(.:format)  things#index

... and when I try to access http://<host>/myengine/things/index, I'll get an uninitialized constant ThingsController.

However, if I change my Engine routes to:

MyEngine::Engine.routes.draw do
  match "things/index" => "things#index", :as => "things_index"
end

... then, rake routes in the dummy application looks like:

$ rake routes
Routes for MyEngine::Engine:
  things_index GET /things/index(.:format)  my_engine/things#index

... and when I try to access http://<host>/myengine/things/index, I get the expected view.

Btw, rails g controller things index will generate a route using the HttpHelper method (not the matcher).

I googled for any corroborating accounts, but didn't see any. So, I'm not sure if this is just something I'm doing wrong. Or, if maybe this is because engines are fairly new.

I'll also post this to stack overflow to see if there's any help there.