Rails generators are an essential tool if you plan to improve your workflow and in this guide you will learn how to create and customize already existing generators.
When you create an application using the +rails+ command, you are in fact using a Rails generator. After that, you can get a list of all available generators by just invoking +rails generate+:
You will get a list of all generators that comes with Rails. If you need a detailed description of the helper generator, for example, you can simply do:
Since Rails 3.0, generators are built on top of "Thor":http://github.com/wycats/thor. Thor provides powerful options parsing and a great API for manipulating files. For instance, let's build a generator that creates an initializer file named +initializer.rb+ inside +config/initializers+.
Our new generator is quite simple: it inherits from +Rails::Generators::Base+ and has one method definition. Each public method in the generator is executed when a generator is invoked. Finally, we invoke the +create_file+ method that will create a file at the given destination with the given content. If you are familiar with the Rails Application Templates API, you'll feel right at home with the new generators API.
Rails is usually able to generate good descriptions if a generator is namespaced, as +ActiveRecord::Generators::ModelGenerator+, but not in this particular case. We can solve this problem in two ways. The first one is calling +desc+ inside our generator:
Now we can see the new description by invoking +--help+ on the new generator. The second way to add a description is by creating a file named +USAGE+ in the same directory as our generator. We are going to do that in the next step.
First, notice that we are inheriting from +Rails::Generators::NamedBase+ instead of +Rails::Generators::Base+. This means that our generator expects as least one argument, which will be the name of the initializer.
We can also see in our new generator that it has a class method called +source_root+. This method points to where our generator templates will be placed and by default it points to the created directory under +RAILS_APP/lib/generators/initializer/templates+. In order to understand what a generator template means, let's create a file at +RAILS_APP/lib/generators/initializer/templates/initializer.rb+ with the following content:
<ruby>
# Add initialization content here
</ruby>
And now let's change the generator to copy this template when invoked:
<ruby>
class InitializerGenerator < Rails::Generators::NamedBase
We can see that now a initializer named foo was created at +config/initializers/foo.rb+ with the contents of our template. That means that +copy_file+ copied a file in our source root to the destination path we gave. The method +file_name+ is automatically created when we inherit from +Rails::Generators::NamedBase+.
Now that we've created our first generator, we need to briefly discuss generator lookup. The way Rails finds generators is exactly the same way Ruby find files, i.e. using +$LOAD_PATHS+.
For instance, when you say +rails generate initializer foo+, Rails knows you want to invoke the initializer generator and then search for the following generators in the $LOAD_PATHS:
Rails generators are flexible enough to let you customize your scaffold the way you want. In your +config/application.rb+ there is a section just for generators:
Looking at this output, it's easy to understand how generators work on Rails 3.0 and above. The scaffold generator doesn't actually generate anything, it just invokes others to do the work. This allows us to add/replace/remove any of those invocations. For instance, the scaffold generator invokes the scaffold_controller generator, which invokes erb, test_unit and helper generators. Since each generator has a single responsibility, they are easy to reuse, avoiding code duplication.
Our first customization on the workflow will be to stop generating stylesheets and test fixtures on scaffold. We can achieve that by changing our application to the following:
If we generate another resource on scaffold, we can notice that neither stylesheets nor fixtures are created anymore. If you want to customize it further, for example to use +Datamapper+ and +RSpec+ instead of +ActiveRecord+ and +TestUnit+, it's just a matter of adding their gems to your application and configuring your generators.
After that, we can delete both the +templates+ directory and the +source_root+ class method from our new generators, because we are not going to need them. So our new generator looks like the following:
We can notice on the output that our new helper was invoked instead of the Rails default. However one thing is missing, which is tests for our new generator and to do that, we are going to reuse old helpers test generators.
Since Rails 3.0, this is easy to do due to the hooks concept. Our new helper does not need to be focused in one specific test framework, it can simply provide a hook and a test framework just needs to implement this hook in order to be compatible.
Now, when the helper generator is invoked and TestUnit is configured as the test framework, it will try to invoke both +MyHelper::Generators::TestUnitGenerator+ and +TestUnit::Generators::MyHelperGenerator+. Since none of those are defined, we can tell our generator to invoke +TestUnit::Generators::HelperGenerator+ instead, which is defined since it's a Rails generator. To do that, we just need to add:
In the step above, we simply wanted to add a line to the generated helper, without adding any extra functionality. There is a simpler way to do that, and it's by replacing the templates of already existing generators.
In Rails 3.0 and above, generators don't just look in the source root for templates, they also search for templates in other paths. And one of them is inside +RAILS_APP/lib/templates+. Since we want to customize +Rails::Generators::HelperGenerator+, we can do that by simply making a template copy inside +RAILS_APP/lib/templates/rails/helper+ with the name +helper.rb+. So let's create that file with the following content:
So now we can revert the changes in +config/application.rb+:
<ruby>
config.generators do |g|
g.orm :active_record
g.template_engine :erb
g.test_framework :test_unit, :fixture => false
g.stylesheets false
end
</ruby>
If you generate another resource, you can see that we got exactly the same result! This is useful if you want to customize your scaffold templates and/or layout by just creating +edit.html.erb+, +index.html.erb+ and so on inside +RAILS_APP/lib/templates/erb/scaffold+.
One last feature about generators which is quite useful for plugin generators is fallbacks. For example, imagine that you want to add a feature on top of TestUnit test framework, like "shoulda":http://github.com/thoughtbot/shoulda does. Since TestUnit already implements all generators required by Rails and shoulda just wants to overwrite part of it, there is no need for shoulda to reimplement some generators again, it can simply tell Rails to use a +TestUnit+ generator if none was found under the +Shoulda+ namespace.
Now, if you create a Comment scaffold, you will see that the shoulda generators are being invoked, and at the end, they are just falling back to test unit generators:
