Creating and Customizing Rails Generators & Templates¶ ↑
Rails generators are an essential tool if you plan to improve your workflow. With this guide you will learn how to create generators and customize existing ones.
After reading this guide, you will know:
-
How to see which generators are available in your application.
-
How to create a generator using templates.
-
How Rails searches for generators before invoking them.
-
How Rails internally generates Rails code from the templates.
-
How to customize your scaffold by creating new generators.
-
How to customize your scaffold by changing generator templates.
-
How to use fallbacks to avoid overwriting a huge set of generators.
-
How to create an application template.
First Contact¶ ↑
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
:
$ rails new myapp $ cd myapp $ bin/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:
$ bin/rails generate helper --help
Creating Your First Generator¶ ↑
Since Rails 3.0, generators are built on top of Thor. Thor provides powerful
options for 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
.
The first step is to create a file at
lib/generators/initializer_generator.rb
with the following
content:
class InitializerGenerator < Rails::Generators::Base def create_initializer_file create_file "config/initializers/initializer.rb", "# Add initialization content here" end end
NOTE: create_file
is a method provided by
Thor::Actions
. Documentation for create_file
and
other Thor methods can be found in Thor’s
documentation
Our new generator is quite simple: it inherits from
Rails::Generators::Base
and has one method definition. When a
generator is invoked, each public method in the generator is executed
sequentially in the order that it is defined. 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.
To invoke our new generator, we just need to do:
$ bin/rails generate initializer
Before we go on, let's see our brand new generator description:
$ bin/rails generate initializer --help
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:
class InitializerGenerator < Rails::Generators::Base desc "This generator creates an initializer file at config/initializers" def create_initializer_file create_file "config/initializers/initializer.rb", "# Add initialization content here" end end
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.
Creating Generators with Generators¶ ↑
Generators themselves have a generator:
$ bin/rails generate generator initializer create lib/generators/initializer create lib/generators/initializer/initializer_generator.rb create lib/generators/initializer/USAGE create lib/generators/initializer/templates
This is the generator just created:
class InitializerGenerator < Rails::Generators::NamedBase source_root File.expand_path("../templates", __FILE__) end
First, notice that we are inheriting from
Rails::Generators::NamedBase
instead of
Rails::Generators::Base
. This means that our generator expects
at least one argument, which will be the name of the initializer, and will
be available in our code in the variable name
.
We can see that by invoking the description of this new generator (don't forget to delete the old generator file):
$ bin/rails generate initializer --help Usage: rails generate initializer NAME [options]
We can also see that our new generator has a class method called
source_root
. This method points to where our generator
templates will be placed, if any, and by default it points to the created
directory lib/generators/initializer/templates
.
In order to understand what a generator template means, let's create
the file lib/generators/initializer/templates/initializer.rb
with the following content:
# Add initialization content here
And now let's change the generator to copy this template when invoked:
class InitializerGenerator < Rails::Generators::NamedBase source_root File.expand_path("../templates", __FILE__) def copy_initializer_file copy_file "initializer.rb", "config/initializers/#{file_name}.rb" end end
And let's execute our generator:
$ bin/rails generate initializer core_extensions
We can see that now an initializer named core_extensions was created at
config/initializers/core_extensions.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
.
The methods that are available for generators are covered in the final section of this guide.
Generators Lookup¶ ↑
When you run rails generate initializer core_extensions
Rails
requires these files in turn until one is found:
rails/generators/initializer/initializer_generator.rb generators/initializer/initializer_generator.rb rails/generators/initializer_generator.rb generators/initializer_generator.rb
If none is found you get an error message.
INFO: The examples above put files under the application's
lib
because said directory belongs to $LOAD_PATH
.
Customizing Your Workflow¶ ↑
Rails own generators are flexible enough to let you customize scaffolding.
They can be configured in config/application.rb
, these are
some defaults:
config.generators do |g| g.orm :active_record g.template_engine :erb g.test_framework :test_unit, fixture: true end
Before we customize our workflow, let's first see what our scaffold looks like:
$ bin/rails generate scaffold User name:string invoke active_record create db/migrate/20130924151154_create_users.rb create app/models/user.rb invoke test_unit create test/models/user_test.rb create test/fixtures/users.yml invoke resource_route route resources :users invoke scaffold_controller create app/controllers/users_controller.rb invoke erb create app/views/users create app/views/users/index.html.erb create app/views/users/edit.html.erb create app/views/users/show.html.erb create app/views/users/new.html.erb create app/views/users/_form.html.erb invoke test_unit create test/controllers/users_controller_test.rb invoke helper create app/helpers/users_helper.rb invoke jbuilder create app/views/users/index.json.jbuilder create app/views/users/show.json.jbuilder invoke assets invoke coffee create app/assets/javascripts/users.js.coffee invoke scss create app/assets/stylesheets/users.css.scss invoke scss create app/assets/stylesheets/scaffolds.css.scss
Looking at this output, it's easy to understand how generators work in 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 stylesheet, JavaScript and test fixture files for scaffolds. We can achieve that by changing our configuration to the following:
config.generators do |g| g.orm :active_record g.template_engine :erb g.test_framework :test_unit, fixture: false g.stylesheets false g.javascripts false end
If we generate another resource with the scaffold generator, we can see that stylesheet, JavaScript and fixture files are not created anymore. If you want to customize it further, for example to use DataMapper and RSpec instead of Active Record and TestUnit, it's just a matter of adding their gems to your application and configuring your generators.
To demonstrate this, we are going to create a new helper generator that simply adds some instance variable readers. First, we create a generator within the rails namespace, as this is where rails searches for generators used as hooks:
$ bin/rails generate generator rails/my_helper create lib/generators/rails/my_helper create lib/generators/rails/my_helper/my_helper_generator.rb create lib/generators/rails/my_helper/USAGE create lib/generators/rails/my_helper/templates
After that, we can delete both the templates
directory and the
source_root
class method call from our new generator, because
we are not going to need them. Add the method below, so our generator looks
like the following:
# lib/generators/rails/my_helper/my_helper_generator.rb class Rails::MyHelperGenerator < Rails::Generators::NamedBase def create_helper_file create_file "app/helpers/#{file_name}_helper.rb", <<-FILE module #{class_name}Helper attr_reader :#{plural_name}, :#{plural_name.singularize} end FILE end end
We can try out our new generator by creating a helper for products:
$ bin/rails generate my_helper products create app/helpers/products_helper.rb
And it will generate the following helper file in app/helpers
:
module ProductsHelper attr_reader :products, :product end
Which is what we expected. We can now tell scaffold to use our new helper
generator by editing config/application.rb
once again:
config.generators do |g| g.orm :active_record g.template_engine :erb g.test_framework :test_unit, fixture: false g.stylesheets false g.javascripts false g.helper :my_helper end
and see it in action when invoking the generator:
$ bin/rails generate scaffold Article body:text [...] invoke my_helper create app/helpers/articles_helper.rb
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.
To do that, we can change the generator this way:
# lib/generators/rails/my_helper/my_helper_generator.rb class Rails::MyHelperGenerator < Rails::Generators::NamedBase def create_helper_file create_file "app/helpers/#{file_name}_helper.rb", <<-FILE module #{class_name}Helper attr_reader :#{plural_name}, :#{plural_name.singularize} end FILE end hook_for :test_framework end
Now, when the helper generator is invoked and TestUnit is configured as the
test framework, it will try to invoke both
Rails::TestUnitGenerator
and
TestUnit::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:
# Search for :helper instead of :my_helper hook_for :test_framework, as: :helper
And now you can re-run scaffold for another resource and see it generating tests as well!
Customizing Your Workflow by Changing Generators Templates¶ ↑
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
that case Rails::Generators::HelperGenerator
.
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 lib/templates
. Since we want to customize
Rails::Generators::HelperGenerator
, we can do that by simply
making a template copy inside lib/templates/rails/helper
with
the name helper.rb
. So let's create that file with the
following content:
module <%= class_name %>Helper attr_reader :<%= plural_name %>, :<%= plural_name.singularize %> end
and revert the last change in config/application.rb
:
config.generators do |g| g.orm :active_record g.template_engine :erb g.test_framework :test_unit, fixture: false g.stylesheets false g.javascripts false end
If you generate another resource, you can see that we get 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
lib/templates/erb/scaffold
.
Scaffold templates in Rails frequently use ERB tags; these tags need to be escaped so that the generated output is valid ERB code.
For example, the following escaped ERB tag would be needed in the template
(note the extra %
)…
<%%= stylesheet_include_tag :application %>
…to generate the following output:
stylesheet_include_tag :application %><%=
Adding Generators Fallbacks¶ ↑
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 like 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.
We can easily simulate this behavior by changing our
config/application.rb
once again:
config.generators do |g| g.orm :active_record g.template_engine :erb g.test_framework :shoulda, fixture: false g.stylesheets false g.javascripts false # Add a fallback! g.fallbacks[:shoulda] = :test_unit end
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 TestUnit generators:
$ bin/rails generate scaffold Comment body:text invoke active_record create db/migrate/20130924143118_create_comments.rb create app/models/comment.rb invoke shoulda create test/models/comment_test.rb create test/fixtures/comments.yml invoke resource_route route resources :comments invoke scaffold_controller create app/controllers/comments_controller.rb invoke erb create app/views/comments create app/views/comments/index.html.erb create app/views/comments/edit.html.erb create app/views/comments/show.html.erb create app/views/comments/new.html.erb create app/views/comments/_form.html.erb invoke shoulda create test/controllers/comments_controller_test.rb invoke my_helper create app/helpers/comments_helper.rb invoke jbuilder create app/views/comments/index.json.jbuilder create app/views/comments/show.json.jbuilder invoke assets invoke coffee create app/assets/javascripts/comments.js.coffee invoke scss
Fallbacks allow your generators to have a single responsibility, increasing code reuse and reducing the amount of duplication.
Application Templates¶ ↑
Now that you've seen how generators can be used inside an application, did you know they can also be used to generate applications too? This kind of generator is referred as a “template”. This is a brief overview of the Templates API. For detailed documentation see the Rails Application Templates guide.
gem "rspec-rails", group: "test" gem "cucumber-rails", group: "test" if yes?("Would you like to install Devise?") gem "devise" generate "devise:install" model_name = ask("What would you like the user model to be called? [user]") model_name = "user" if model_name.blank? generate "devise", model_name end
In the above template we specify that the application relies on the
rspec-rails
and cucumber-rails
gem so these two
will be added to the test
group in the Gemfile
.
Then we pose a question to the user about whether or not they would like to
install Devise. If the user replies “y” or “yes” to this question, then the
template will add Devise to the Gemfile
outside of any group
and then runs the devise:install
generator. This template then
takes the users input and runs the devise
generator, with the
user's answer from the last question being passed to this generator.
Imagine that this template was in a file called template.rb
.
We can use it to modify the outcome of the rails new
command
by using the -m
option and passing in the filename:
$ rails new thud -m template.rb
This command will generate the Thud
application, and then
apply the template to the generated output.
Templates don't have to be stored on the local system, the
-m
option also supports online templates:
$ rails new thud -m https://gist.github.com/radar/722911/raw/
Whilst the final section of this guide doesn't cover how to generate the most awesome template known to man, it will take you through the methods available at your disposal so that you can develop it yourself. These same methods are also available for generators.
Generator methods¶ ↑
The following are methods available for both generators and templates for Rails.
NOTE: Methods provided by Thor are not covered this guide and can be found in Thor’s documentation
gem
¶ ↑
Specifies a gem dependency of the application.
gem "rspec", group: "test", version: "2.1.0" gem "devise", "1.1.5"
Available options are:
-
:group
- The group in theGemfile
where this gem should go. -
:version
- The version string of the gem you want to use. Can also be specified as the second argument to the method. -
:git
- The URL to the git repository for this gem.
Any additional options passed to this method are put on the end of the line:
gem "devise", git: "git://github.com/plataformatec/devise", branch: "master"
The above code will put the following line into Gemfile
:
gem "devise", git: "git://github.com/plataformatec/devise", branch: "master"
gem_group
¶ ↑
Wraps gem entries inside a group:
gem_group :development, :test do gem "rspec-rails" end
add_source
¶ ↑
Adds a specified source to Gemfile
:
add_source "http://gems.github.com"
inject_into_file
¶ ↑
Injects a block of code into a defined position in your file.
inject_into_file 'name_of_file.rb', after: "#The code goes below this line. Don't forget the Line break at the end\n" do <<-'RUBY' puts "Hello World" RUBY end
gsub_file
¶ ↑
Replaces text inside a file.
gsub_file 'name_of_file.rb', 'method.to_be_replaced', 'method.the_replacing_code'
Regular Expressions can be used to make this method more precise. You can
also use append_file
and prepend_file
in the same
way to place code at the beginning and end of a file respectively.
application
¶ ↑
Adds a line to config/application.rb
directly after the
application class definition.
application "config.asset_host = 'http://example.com'"
This method can also take a block:
application do "config.asset_host = 'http://example.com'" end
Available options are:
-
:env
- Specify an environment for this configuration option. If you wish to use this option with the block syntax the recommended syntax is as follows:
application(nil, env: "development") do "config.asset_host = 'http://localhost:3000'" end
git
¶ ↑
Runs the specified git command:
git :init git add: "." git commit: "-m First commit!" git add: "onefile.rb", rm: "badfile.cxx"
The values of the hash here being the arguments or options passed to the specific git command. As per the final example shown here, multiple git commands can be specified at a time, but the order of their running is not guaranteed to be the same as the order that they were specified in.
vendor
¶ ↑
Places a file into vendor
which contains the specified code.
vendor "sekrit.rb", '#top secret stuff'
This method also takes a block:
vendor "seeds.rb" do "puts 'in your app, seeding your database'" end
lib
¶ ↑
Places a file into lib
which contains the specified code.
lib "special.rb", "p Rails.root"
This method also takes a block:
lib "super_special.rb" do puts "Super special!" end
rakefile
¶ ↑
Creates a Rake file in the lib/tasks
directory of the
application.
rakefile "test.rake", "hello there"
This method also takes a block:
rakefile "test.rake" do %Q{ task rock: :environment do puts "Rockin'" end } end
initializer
¶ ↑
Creates an initializer in the config/initializers
directory of
the application:
initializer "begin.rb", "puts 'this is the beginning'"
This method also takes a block, expected to return a string:
initializer "begin.rb" do "puts 'this is the beginning'" end
generate
¶ ↑
Runs the specified generator where the first argument is the generator name and the remaining arguments are passed directly to the generator.
generate "scaffold", "forums title:string description:text"
rake
¶ ↑
Runs the specified Rake task.
rake "db:migrate"
Available options are:
-
:env
- Specifies the environment in which to run this rake task. -
:sudo
- Whether or not to run this task usingsudo
. Defaults tofalse
.
capify!
¶ ↑
Runs the capify
command from Capistrano at the root of the
application which generates Capistrano configuration.
capify!
route
¶ ↑
Adds text to the config/routes.rb
file:
route "resources :people"
readme
¶ ↑
Output the contents of a file in the template's
source_path
, usually a README.
readme "README"