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= Welcome

This document gives an overview of how the Engines mechanism works within a Rails environment. In most cases the code below is just an example. For more information or documentation, please go to http://rails-engines.org.

== Background

Rails Engines are a way of dropping in whole chunks of functionality into your
existing application without affecting *any* of your existing code. The could also be described as mini-applications, or vertical application slices - top-to-bottom units which provide full MVC coverage for a certain, specific application function.

As an example, the Login Engine provides a full user login subsystem, including: 
* controllers to manage user accounts; 
* helpers for you to interact with account information from other 
  parts of your application; 
* the model objects and schemas to create the required tables; 
* stylesheets and javascript files to enhance the views; 
* and any other library files required.

Once the Rails Core team decides on a suitable method for packaging plugins, Engines can be distributed using the same mechanisms. If you are developing engines yourself for use across multiple projects, linking them as svn externals allows seamless updating of bugfixes across multiple applications.





= Edge Engines

If you are using Edge Rails (an SVN copy of Rails, rather than an 'official' release), there are several MASSIVELY IMPORTANT issues that you need to bear in mind.

Firstly, you are using an unstable version of Rails, so it is possible that things will break. We work hard to keep the Engines plugin up to speed with the changes in Rails at the bleeding edge, but sometimes significant parts of Rails change it WILL cause problems. This is the price of using the bleeding edge. Since edge is synonymous for unstable, the version of the Engines plugin which is compatible with Edge Rails is kept separate from the official release. Please ensure that you have used SVN to get your engines plugin from

  http://svn.rails-engines.org/engines/trunk
  
The normal 'script/plugin install engines' will NOT get you this version.

Secondly, you NEED to tell the engines plugin if you expect it to perform with Edge behaviour. This is done by adding the following lines at the *very top* of environment.rb (yes, the VERY TOP)

  module Engines
    CONFIG = {:edge => true}
  end

This will set the plugin to work with Edge Rails, rather than expecting an official release.

If you are having problems, please try and contribute a bug report if you can so we can improve the plugin and keep up to speed with Rails' bleeding edge. Your input is *absolutely crucial* to this. If you're not comfortable with tracking down bugs in Rails' and Engines' internal code, there is a test application available at

  http://svn.rails-engines.org/applications/engines_test
  
which contains an array of tests that might help you (and us) pinpoint where the issue is. Please download this application and consult the README for more information.

Finally, please don't forget about our website and mailing lists. More information here:

  http://rails-engines.org







= Using the Engines plugin itself

There are a number of features of the Engines plugin itself which may be useful to know:


=== Engines.log
The Engines plugin comes with its own logger, which is invaluable when debugging. To use it,
simply call

  Engines.create_logger
  
Two optional arguments may be passed to this method:

  Engines.create_logger(<io>)
  
Would set the outputter to the logger to the given IO object <io>. For example, this could be STDERR or STDOUT (the default). The second argument is the logger level:

  Engines.create_logger(STDOUT, Logger::INFO)
  
The logger can be accessed using either of the following:

  Engines.log.[debug|info|whatever] "message"
  Engines.logger.[debug|info|whatever] "message"
  
... essentially it's a Logger object. It's worth noting that if you *don't* create a logger, calls to Engines.log will just be swallowed without a sound, making it very very easy to completely silence Engine logging.


=== Engines.config(:root)

By default, the Engines plugin expects to be starting Engines from within RAILS_ROOT/vendor/plugins. However, if you'd like to store your engines in a different directory, add the following line *before* any call to Engines.start

  Engines.config(:root, "/path/to/your/directory", :force)
  
  
=== Rake Tasks

The engines plugin comes with a number of handy rake tasks:

  # display version information about the engines subsystem
  rake engines:info 
  
  # migrate engines' database schemas in a controlled way
  rake db:migrate:engines
  
  # generates full documentation for all engines
  rake doc:engines      
  
There are more, but you'll have to discover them yourself...  


== More information

For more information about what you can do with the Engines plugin, you'll need to generate the documentation (rake plugindoc), or go to http://rails-engines.org. Good luck!  








= Quickstart

=== Gentlemen, Start your Engines!


Here's an *example* of how you might go about using Rails Engines. Please bear in mind that actual Engines may differ from this, but these are the steps you will *typically* have to take. Refer to individual Engine documentation for specific installation instructions. Anyway, on with the show:

1. Install the Rails Engines plugin into your plugins directory. You'll probably need to accept the SSL certificate here for the OpenSVN servers. For example:

    $ script/plugin install engines

 or

    $ svn co http://svn.rails-engines.org/plugins/engines <MY_RAILS_APP>/vendor/plugins/engines

2. Install your engine into the plugins directory in a similar way.

3. Create the RDoc for the engines plugin and for your engines so you know what's going on:

     $ rake doc:plugins
     $ rake doc:engines

4. Initialize any database schema provided. The Engine may provide Rake tasks to do this for you. Beware that accepting an Engine schema might affect any existing database tables you have installed! You are STRONGLY recommended to inspect the <tt>db/schema.rb</tt> file to see exactly what running it might change.

5. Add configuration to <tt>environment.rb</tt>:
   e.g.

      # Add your application configuration here
      module MyEngine
        config :top_speed, "MegaTurboFast"
      end
 
      Engines.start :my_engine

6. Run your server!

      $ script/server




= Building an Engine

Here's a sample rails application with a detailed listing of an example engines as a concrete example:

  RAILS_ROOT
    |- app
    |- lib
    |- config
    |- <... other directories ...>
    |- vendor
        |-plugins
            |- engines               <-- the engines plugin
            |- some_other_plugin 
            |- my_engine             <-- our example engine
                  |- init_engine.rb
                  |- app
                  |     |- controllers
                  |     |- models
                  |     |- helpers
                  |     |- views
                  |- db
                  |- tasks
                  |- lib
                  |- public
                  |     |- javascripts
                  |     |- stylesheets
                  |- test


The internal structure of an engine mirrors the familiar core of a Rails application, with most of the engine within the <tt>app</tt> subdirectory. Within <tt>app</tt>, the controllers, views and model objects behave just as you might expect if there in the top-level <tt>app</tt> directory.

When you call <tt>Engines.start :my_engine</tt> in <tt>environment.rb</tt> a few important bits of black magic voodoo happen:
* the engine's controllers, views and models are mixed in to your running Rails application; 
* files in the <tt>lib</tt> directory of your engine (and subdirectories) are made available 
  to the rest of your system
* any directory structure in the folder <tt>public/</tt> within your engine is made available to the webserver
* the file <tt>init_engine.rb</tt> is loaded from within the engine - just like a plugin. The reason why engines need an init_engine.rb rather than an init.rb is because Rails' default plugin system might try and load an engine before the Engines plugin has been loaded, resulting in all manner of badness. Instead, Rails' skips over any engine plugins, and the Engines plugin handles initializing your Engines plugins when you 'start' each engine.

From within <tt>init_engine.rb</tt> you should load any libraries from your <tt>lib</tt> directory that your engine might need to function. You can also perform any configuration required.

=== Loading all Engines

Calling either Engines.start (with no arguments) or Engines.start_all will load all engines available. Please note that your plugin can only be *automatically* detected as an engine by the presence of an 'init_engine.rb' file, or if the engine is in a directory named <something>_engine, or <something>_bundle. If neither of these conditions hold, then your engine will not be loaded by Engines.start() (with no arguments) or Engines.start_all().







= Configuring Engines

Often your engine will require a number of configuration parameters set, some of which should be alterable by the user to reflect their particular needs. For example, a Login System might need a unique Salt value set to encrypt user passwords. This value should be unique to each application.

Engines provides a simple mechanism to handle this, and it's already been hinted at above. Within any module, a new method is now available: <tt>config</tt>. This method creates a special <tt>CONFIG</tt> Hash object within the Module it is called, and can be used to store your parameters. For a user to set these parameters, they should reopen the module (before the corresponding Engines.start call), as follows:

  module MyModule
    config :some_option, "really_important_value"
  end
  Engines.start :my_engine

Because this config value has been set before the Engine is started, subsequent attempts to set this config value will be ignored and the user-specified value used instead. Of course, there are situations where you *really* want to set the config value, even if it already exists. In such cases the config call can be changed to:

  config :some_option, "no_THIS_really_important_value", :force

The additional parameter will force the new value to be used. For more information, see Module#config.




= Tweaking Engines

One of the best things about Engines is that if you don't like the default behaviour of any component, you can override it without needing to overhaul the whole engine. This makes adding your customisations to engines almost painless, and allows for upgrading/updating engine code without affecting the specialisations you need for your particular application.


=== View Tweaks
These are the simplest - just drop your customised view (or partial) into you <tt>/app/views</tt> directory in the corresponding location for the engine, and your view will be used in preference to the engine view. For example, if we have a ItemController with an action 'show', it will (normally) expect to find its view as <tt>report/show.rhtml</tt> in the <tt>views</tt> directory. The view is found in the engine at <tt>/vendor/engines/my_engine/app/views/report/show.rhtml</tt>. However, if you create the file <tt>/app/views/report/show.rhtml</tt>, that file will be used instead! The same goes for partials.


=== Controller Tweaks
You can override controller behaviour by replacing individual controller methods with your custom behaviour. Lets say that our ItemController's 'show' method isn't up to scratch, but the rest of it behaves just fine. To override the single method, create <tt>/app/controllers/item_controller.rb</tt>, just as if it were going to be a new controller in a regular Rails application. then, implement your show method as you would like it to happen.

... and that's it. Your custom code will be mixed in to the engine controller, replacing its old method with your custom code.


=== Model/Lib Tweaks
Alas, tweaking model objects isn't quite so easy (yet). If you need to change the behaviour of model objects, you'll need to copy the model file from the engine into <tt>/app/models</tt> and edit the methods yourself. Library code can be overridden in a similar way.









= Public Files

If the Engine includes a <tt>public</tt> directory, its contents will be mirrored into <tt>RAILS_ROOT/public/engine_files/&lt;engine_name&gt;/</tt> so that these files can be served by your webserver to browsers and users over the internet.

Engines also provides two convenience methods for loading stylesheets and javascript files in your layouts: <tt>engine_stylesheet</tt> and <tt>engine_javascript</tt>.

=== Engine Stylesheets

  <%= engine_stylesheet "my_engine" %>

will include <tt>RAILS_ROOT/public/&lt;engine_files&gt;/my_engine/stylesheets/my_engine.css</tt> in your layout. If you have more than one stylesheet, you can include them in the same call:

  <%= engine_stylesheet "my_engine", "stylesheet_2", "another_one" %>

will give you:

    <link href="/engine_files/my_engine/stylesheets/my_engine.css" media="screen" rel="Stylesheet" type="text/css" />
    <link href="/engine_files/my_engine/stylesheets/stylesheet_2.css" media="screen" rel="Stylesheet" type="text/css" />
    <link href="/engine_files/my_engine/stylesheets/another_one.css" media="screen" rel="Stylesheet" type="text/css" />

in your rendered layout.

=== Engine Javascripts

The <tt>engine_javascript</tt> command works in exactly the same way as above.









= Databases and Engines

Engine schema information can be handled using similar mechanisms to your normal application schemas.

== CAVEAT EMPTOR!

I.E. Big Huge Warning In Flashing Lights.

PLEASE, PLEASE, PLEASE bear in mind that if you are letting someone
ELSE have a say in what tables you are using, you're basically opening
your application schema open to potential HAVOC. I cannot stress how
serious this is. It is YOUR responsibility to ensure that you trust
the schema and migration information, and that it's not going to drop
your whole database. You need to inspect these things. YOU do. If you
run these voodoo commands and your database essplodes because you
didn't double double double check what was going on, your embarassment
will be infinite. And your project will be skroo'd if the migration
is irreversible.

That said, if you are working in a team and you all trust each other,
which is hopefully true, this can be quite useful.


== Migrating Engines

To migrate all engines to the latest version:

 rake db:migrate:engines

To migrate a single engine, for example the login engine:

 rake db:migrate:engines ENGINE=login  (or login_engine)

To migrate a single engine to a specific revision:

 rake db:migrate:engines ENGINE=login VERSION=4

This:

 rake db:migrate:engines VERSION=1

... will not work, because we felt it was too dangerous to allow ALL
engines to be migrated to a specific version, since such versions
might be incompatible.







= Testing Engines

The Engines plugin comes with mechanisms to help test Engines within a developer's own application. The testing extensions enable developers to load fixtures into specific
tables irrespective of the name of the fixtures file. This work is heavily based on
patches made by Duane Johnson (canadaduane), viewable at
http://dev.rubyonrails.org/ticket/1911

Engine developers should supply fixture files in the <engine>/test/fixtures directory
as normal. Within their tests, they should load the fixtures using the 'fixture' command
(rather than the normal 'fixtures' command). For example:

  class UserTest < Test::Unit::TestCase
    fixture :users, :table_name => LoginEngine.config(:user_table), :class_name => "User"
 
    ...

This will ensure that the fixtures/users.yml file will get loaded into the correct
table, and will use the correct model object class.

Your engine should provide a test_helper.rb file in <engine>/test, the contents of which should include the following:

  # Load the default rails test helper - this will load the environment.
        require File.dirname(__FILE__) + '/../../../../test/test_helper'

        # ensure that the Engines testing enhancements are loaded and will override Rails own
        # code where needed. This line is very important!
        require File.join(Engines.config(:root), "engines", "lib", "testing_extensions")

        # Load the schema - if migrations have been performed, this will be up to date.
        load(File.dirname(__FILE__) + "/../db/schema.rb")

        # set up the fixtures location to use your engine's fixtures
        Test::Unit::TestCase.fixture_path = File.dirname(__FILE__)  + "/fixtures/"
        $LOAD_PATH.unshift(Test::Unit::TestCase.fixture_path)

== Loading Fixtures

An additional helpful task for loading fixture data is also provided (thanks to Joe Van Dyk):

  rake db:fixtures:engines:load
  rake db:fixtures:engines:load PLUGIN=login_engine
  
will load the engine fixture data into your development database.

=== Important Caveat
Unlike the new 'fixture' directive described above, this task currently relies on you ensuring that the table name to load fixtures into is the same as the name of the fixtures file you are trying to load. If you are using defaults, this should be fine. If you have changed table names, you will need to rename your fixtures files (and possibly update your tests to reflect this too).

You should also note that fixtures typically tend to depend on test configuration information (such as test salt values), so not all data will be usable in fixture form.



= LICENCE

Copyright (c) 2006, James Adam

The MIT License

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.