Rails%203%20In%20Action

514 CHAPTER 17 Engines
This task actually copies across the engines migration into the application’s db/
migrate directory, with the first migration having a timestamp of the current second,
the next migration having it one second in the future, and so on. This is so the
engine’s migrations are inserted after the current application’s migrations and maintain
the same order that they have in the engine. If you update the engine later on,
you’ll have to run this task again. To install these migrations, you’ll run this:
rake db:migrate
After this, you’ll need to mount the engine in your application’s config/routes.rb,
which you can do with this:
mount Forem::Engine, :at => "/forem"
You saw this line in your spec/dummy/config/routes.rb file earlier. Mounting your
engine will make its routes available to the specified path. If you launch rails server
now and go to http://localhost:3000, you’ll be shown this error:
Please define Forem::Engine.user_class in config/initializers/forem.rb
You should follow the advice for this error and define Forem::Engine.user_class
within the application’s config/initializers/forem.rb, as you need to tell the engine
what the user class is:
Forem::Engine.user_class = User
Because you’ve created a new initializer, you’ll need to restart the rails server that is
currently running so that Rails re-evaluates the files in the config/intializers directory.
When you do this and refresh the page, you’ll see this error:
undefined local variable or method `admin_root_path' ...
This is happening because your engine is using the application’s layout and is trying
to reference admin_root_path method from inside the engine, rather than the one
that’s defined in the application. To fix this, you’ll need to first call main_app for these
routing helpers and then call the helpers on that. You need to change the root_path,
admin_root_path, destroy_user_session_path, new_user_registration_path, and
new_user_session_path helpers in the app/views/layouts/application.html.erb file to
all have the main_app. prefix on them.
These are all the changes you need to make in order to integrate your engine with
your application. Click around a bit and try it out!
17.10 Summary
In this very long chapter we’ve covered quite the gamut! We’ve gone through the theory
first of all, the history of engines, and why they’re useful. After that, you saw the
layout of one and how the routing for an engine works.
You then spent the rest of the chapter creating your own engine called forem,
which provides the basic functionality of a forum: topics and posts.