Rails Application Templates

Application templates are simple Ruby files containing DSL for adding gems, initializers, etc. to your freshly created Rails project or an existing Rails project.

After reading this guide, you will know:

1 Usage

To apply a template, you need to provide the Rails generator with the location of the template you wish to apply using the -m option. This can either be a path to a file or a URL.

$ rails new blog -m ~/template.rb
$ rails new blog -m http://example.com/template.rb

You can use the app:template rails command to apply templates to an existing Rails application. The location of the template needs to be passed in via the LOCATION environment variable. Again, this can either be path to a file or a URL.

$ bin/rails app:template LOCATION=~/template.rb
$ bin/rails app:template LOCATION=http://example.com/template.rb

2 Template API

The Rails templates API is easy to understand. Here's an example of a typical Rails template:

# template.rb
generate(:scaffold, "person name:string")
route "root to: 'people#index'"
rails_command("db:migrate")

after_bundle do
  git :init
  git add: "."
  git commit: %Q{ -m 'Initial commit' }
end

The following sections outline the primary methods provided by the API:

2.1 gem(*args)

Adds a gem entry for the supplied gem to the generated application's Gemfile.

For example, if your application depends on the gems bj and nokogiri:

gem "bj"
gem "nokogiri"

Note that this method only adds the gem to the Gemfile; it does not install the gem.

You can also specify an exact version:

gem "nokogiri", "~> 1.16.4"

And you can also add comments that will be added to the Gemfile:

gem "nokogiri", "~> 1.16.4", comment: "Add the nokogiri gem for XML parsing"

2.2 gem_group(*names, &block)

Wraps gem entries inside a group.

For example, if you want to load rspec-rails only in the development and test groups:

gem_group :development, :test do
  gem "rspec-rails"
end

2.3 add_source(source, options={}, &block)

Adds the given source to the generated application's Gemfile.

For example, if you need to source a gem from "http://gems.github.com":

add_source "http://gems.github.com"

If block is given, gem entries in block are wrapped into the source group.

add_source "http://gems.github.com/" do
  gem "rspec-rails"
end

2.4 environment/application(data=nil, options={}, &block)

Adds a line inside the Application class for config/application.rb.

If options[:env] is specified, the line is appended to the corresponding file in config/environments.

environment 'config.action_mailer.default_url_options = {host: "http://yourwebsite.example.com"}', env: 'production'

A block can be used in place of the data argument.

2.5 vendor/lib/file/initializer(filename, data = nil, &block)

Adds an initializer to the generated application's config/initializers directory.

Let's say you like using Object#not_nil? and Object#not_blank?:

initializer 'bloatlol.rb', <<-CODE
  class Object
    def not_nil?
      !nil?
    end

    def not_blank?
      !blank?
    end
  end
CODE

Similarly, lib() creates a file in the lib/ directory and vendor() creates a file in the vendor/ directory.

There is even file(), which accepts a relative path from Rails.root and creates all the directories/files needed:

file 'app/components/foo.rb', <<-CODE
  class Foo
  end
CODE

That'll create the app/components directory and put foo.rb in there.

2.6 rakefile(filename, data = nil, &block)

Creates a new rake file under lib/tasks with the supplied tasks:

rakefile("bootstrap.rake") do
  <<-TASK
    namespace :boot do
      task :strap do
        puts "i like boots!"
      end
    end
  TASK
end

The above creates lib/tasks/bootstrap.rake with a boot:strap rake task.

2.7 generate(what, *args)

Runs the supplied rails generator with given arguments.

generate(:scaffold, "person", "name:string", "address:text", "age:number")

2.8 run(command)

Executes an arbitrary command. Just like the backticks. Let's say you want to remove the README.rdoc file:

run "rm README.rdoc"

2.9 rails_command(command, options = {})

Runs the supplied command in the Rails application. Let's say you want to migrate the database:

rails_command "db:migrate"

You can also run commands with a different Rails environment:

rails_command "db:migrate", env: 'production'

You can also run commands as a super-user:

rails_command "log:clear", sudo: true

You can also run commands that should abort application generation if they fail:

rails_command "db:migrate", abort_on_failure: true

2.10 route(routing_code)

Adds a routing entry to the config/routes.rb file. In the steps above, we generated a person scaffold and also removed README.rdoc. Now, to make PeopleController#index the default page for the application:

route "root to: 'person#index'"

2.11 inside(dir)

Enables you to run a command from the given directory. For example, if you have a copy of edge rails that you wish to symlink from your new apps, you can do this:

inside('vendor') do
  run "ln -s ~/commit-rails/rails rails"
end

2.12 ask(question)

ask() gives you a chance to get some feedback from the user and use it in your templates. Let's say you want your user to name the new shiny library you're adding:

lib_name = ask("What do you want to call the shiny library ?")
lib_name << ".rb" unless lib_name.index(".rb")

lib lib_name, <<-CODE
  class Shiny
  end
CODE

2.13 yes?(question) or no?(question)

These methods let you ask questions from templates and decide the flow based on the user's answer. Let's say you want to prompt the user to run migrations:

rails_command("db:migrate") if yes?("Run database migrations?")
# no?(question) acts just the opposite.

2.14 git(:command)

Rails templates let you run any git command:

git :init
git add: "."
git commit: "-a -m 'Initial commit'"

2.15 after_bundle(&block)

Registers a callback to be executed after the gems are bundled and binstubs are generated. Useful for adding generated files to version control:

after_bundle do
  git :init
  git add: '.'
  git commit: "-a -m 'Initial commit'"
end

The callbacks gets executed even if --skip-bundle has been passed.

3 Advanced Usage

The application template is evaluated in the context of a Rails::Generators::AppGenerator instance. It uses the apply action provided by Thor.

This means you can extend and change the instance to match your needs.

For example by overwriting the source_paths method to contain the location of your template. Now methods like copy_file will accept relative paths to your template's location.

def source_paths
  [__dir__]
end

Feedback

You're encouraged to help improve the quality of this guide.

Please contribute if you see any typos or factual errors. To get started, you can read our documentation contributions section.

You may also find incomplete content or stuff that is not up to date. Please do add any missing documentation for main. Make sure to check Edge Guides first to verify if the issues are already fixed or not on the main branch. Check the Ruby on Rails Guides Guidelines for style and conventions.

If for whatever reason you spot something to fix but cannot patch it yourself, please open an issue.

And last but not least, any kind of discussion regarding Ruby on Rails documentation is very welcome on the official Ruby on Rails Forum.