Creating and Customizing Rails Generators & Templates

1. 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 invoking bin/rails generate:

$ rails new myapp
$ cd myapp
$ bin/rails generate

You will get a list of all generators that come with Rails. To see a detailed description of a particular generator, invoke the generator with the --help option. For example:

$ bin/rails generate scaffold --help

2. Creating Your First Generator

Generators are built on top of Thor, which provides powerful options for parsing and a great API for manipulating files.

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", <<~RUBY
      # Add initialization content here
    RUBY
  end
end

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. Our method invokes create_file, which will create a file at the given destination with the given content.

To invoke our new generator, we run:

$ bin/rails generate initializer

Before we go on, let's see the description of our new generator:

$ bin/rails generate initializer --help

Rails is usually able to derive a good description if a generator is namespaced, such as ActiveRecord::Generators::ModelGenerator, but not in this case. We can solve this problem in two ways. The first way to add a description is by 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", <<~RUBY
      # Add initialization content here
    RUBY
  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.

3. Creating Generators with Generators

Generators themselves have a generator. Let's remove our InitializerGenerator and use bin/rails generate generator to generate a new one:

$ rm lib/generators/initializer_generator.rb

$ 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
      invoke  test_unit
      create    test/lib/generators/initializer_generator_test.rb

This is the generator just created:

class InitializerGenerator < Rails::Generators::NamedBase
  source_root File.expand_path("templates", __dir__)
end

First, notice that the generator inherits 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 to our code via name.

We can see that by checking the description of the new generator:

$ bin/rails generate initializer --help
Usage:
  bin/rails generate initializer NAME [options]

Also, notice that the generator has a class method called source_root. This method points to the location of our templates, if any. By default it points to the lib/generators/initializer/templates directory that was just created.

In order to understand how generator templates work, let's create the file lib/generators/initializer/templates/initializer.rb with the following content:

# Add initialization content here

And let's change the generator to copy this template when invoked:

class InitializerGenerator < Rails::Generators::NamedBase
  source_root File.expand_path("templates", __dir__)

  def copy_initializer_file
    copy_file "initializer.rb", "config/initializers/#{file_name}.rb"
  end
end

Now let's run our generator:

$ bin/rails generate initializer core_extensions
      create  config/initializers/core_extensions.rb

$ cat config/initializers/core_extensions.rb
# Add initialization content here

We see that copy_file created config/initializers/core_extensions.rb with the contents of our template. (The file_name method used in the destination path is inherited from Rails::Generators::NamedBase.)

4. Generator Command Line Options

Generators can support command line options using class_option. For example:

class InitializerGenerator < Rails::Generators::NamedBase
  class_option :scope, type: :string, default: "app"
end

Now our generator can be invoked with a --scope option:

$ bin/rails generate initializer theme --scope dashboard

Option values are accessible in generator methods via options:

def copy_initializer_file
  @scope = options["scope"]
end

5. Generator Resolution

When resolving a generator's name, Rails looks for the generator using multiple file names. For example, when you run bin/rails generate initializer core_extensions, Rails tries to load each of the following files, in order, 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 of these are found, an error will be raised.

We put our generator in the application's lib/ directory because that directory is in $LOAD_PATH, thus allowing Rails to find and load the file.

6. Overriding Rails Generator Templates

Rails will also look in multiple places when resolving generator template files. One of those places is the application's lib/templates/ directory. This behavior allows us to override the templates used by Rails' built-in generators. For example, we could override the scaffold controller template or the scaffold view templates.

To see this in action, let's create a lib/templates/erb/scaffold/index.html.erb.tt file with the following contents:

<%% @<%= plural_table_name %>.count %> <%= human_name.pluralize %>

Note that the template is an ERB template that renders another ERB template. So any <% that should appear in the resulting template must be escaped as <%% in the generator template.

Now let's run Rails' built-in scaffold generator:

$ bin/rails generate scaffold Post title:string
      ...
      create      app/views/posts/index.html.erb
      ...

The contents of app/views/posts/index.html.erb is:

<% @posts.count %> Posts

7. Overriding Rails Generators

Rails' built-in generators can be configured via config.generators, including overriding some generators entirely.

First, let's take a closer look at how the scaffold generator works.

$ bin/rails generate scaffold User name:string
      invoke  active_record
      create    db/migrate/20230518000000_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
      create      app/views/users/_user.html.erb
      invoke    resource_route
      invoke    test_unit
      create      test/controllers/users_controller_test.rb
      create      test/system/users_test.rb
      invoke    helper
      create      app/helpers/users_helper.rb
      invoke      test_unit
      invoke    jbuilder
      create      app/views/users/index.json.jbuilder
      create      app/views/users/show.json.jbuilder

From the output, we can see that the scaffold generator invokes other generators, such as the scaffold_controller generator. And some of those generators invoke other generators too. In particular, the scaffold_controller generator invokes several other generators, including the helper generator.

Let's override the built-in helper generator with a new generator. We'll name the generator my_helper:

$ 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
      invoke  test_unit
      create    test/lib/generators/rails/my_helper_generator_test.rb

And in lib/generators/rails/my_helper/my_helper_generator.rb we'll define the generator as:

class Rails::MyHelperGenerator < Rails::Generators::NamedBase
  def create_helper_file
    create_file "app/helpers/#{file_name}_helper.rb", <<~RUBY
      module #{class_name}Helper
        # I'm helping!
      end
    RUBY
  end
end

Finally, we need to tell Rails to use the my_helper generator instead of the built-in helper generator. For that we use config.generators. In config/application.rb, let's add:

config.generators do |g|
  g.helper :my_helper
end

Now if we run the scaffold generator again, we see the my_helper generator in action:

$ bin/rails generate scaffold Article body:text
      ...
      invoke  scaffold_controller
      ...
      invoke    my_helper
      create      app/helpers/articles_helper.rb
      ...

7.1. Generators Fallbacks

Another way to override specific generators is by using fallbacks. A fallback allows a generator namespace to delegate to another generator namespace.

For example, let's say we want to override the test_unit:model generator with our own my_test_unit:model generator, but we don't want to replace all of the other test_unit:* generators such as test_unit:controller.

First, we create the my_test_unit:model generator in lib/generators/my_test_unit/model/model_generator.rb:

module MyTestUnit
  class ModelGenerator < Rails::Generators::NamedBase
    source_root File.expand_path("templates", __dir__)

    def do_different_stuff
      say "Doing different stuff..."
    end
  end
end

Next, we use config.generators to configure the test_framework generator as my_test_unit, but we also configure a fallback such that any missing my_test_unit:* generators resolve to test_unit:*:

config.generators do |g|
  g.test_framework :my_test_unit, fixture: false
  g.fallbacks[:my_test_unit] = :test_unit
end

Now when we run the scaffold generator, we see that my_test_unit has replaced test_unit, but only the model tests have been affected:

$ bin/rails generate scaffold Comment body:text
      invoke  active_record
      create    db/migrate/20230518000000_create_comments.rb
      create    app/models/comment.rb
      invoke    my_test_unit
    Doing different stuff...
      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
      create      app/views/comments/_comment.html.erb
      invoke    resource_route
      invoke    my_test_unit
      create      test/controllers/comments_controller_test.rb
      create      test/system/comments_test.rb
      invoke    helper
      create      app/helpers/comments_helper.rb
      invoke      my_test_unit
      invoke    jbuilder
      create      app/views/comments/index.json.jbuilder
      create      app/views/comments/show.json.jbuilder

8. Application Templates

Application templates are a special kind of generator. They can use all of the generator helper methods, but are written as a Ruby script instead of a Ruby class. Here is an example:

# template.rb

if yes?("Would you like to install Devise?")
  gem "devise"
  devise_model = ask("What would you like the user model to be called?", default: "User")
end

after_bundle do
  if devise_model
    generate "devise:install"
    generate "devise", devise_model
    rails_command "db:migrate"
  end

  git add: ".", commit: %(-m 'Initial commit')
end

First, the template asks the user whether they would like to install Devise. If the user replies "yes" (or "y"), the template adds Devise to the Gemfile, and asks the user for the name of the Devise user model (defaulting to User). Later, after bundle install has been run, the template will run the Devise generators and rails db:migrate if a Devise model was specified. Finally, the template will git add and git commit the entire app directory.

We can run our template when generating a new Rails application by passing the -m option to the rails new command:

$ rails new my_cool_app -m path/to/template.rb

Alternatively, we can run our template inside an existing application with bin/rails app:template:

$ bin/rails app:template LOCATION=path/to/template.rb

Templates also don't need to be stored locally — you can specify a URL instead of a path:

$ rails new my_cool_app -m http://example.com/template.rb
$ bin/rails app:template LOCATION=http://example.com/template.rb

9. Generator Helper Methods

Thor provides many generator helper methods via Thor::Actions, such as:

• copy_file
• create_file
• gsub_file
• insert_into_file
• inside

In addition to those, Rails also provides many helper methods via Rails::Generators::Actions, such as:

• environment
• gem
• generate
• git
• initializer
• lib
• rails_command
• rake
• route

10. Testing Generators

Rails provides testing helper methods via Rails::Generators::Testing::Behaviour, such as:

• run_generator

If running tests against generators you will need to set RAILS_LOG_TO_STDOUT=true in order for debugging tools to work.

RAILS_LOG_TO_STDOUT=true ./bin/test test/generators/actions_test.rb

In addition to those, Rails also provides additional assertions via Rails::Generators::Testing::Assertions.