example for https://github.com/alexrothenberg/ammeter/issues/16

config_generator.rb

module Sth
  module Generators
    class ConfigGenerator < ::Rails::Generators::Base
      TXT = '# some inserted text'

      desc 'Configs application controller'
      def edit_application_Controller
        if File.exist?("app/controllers/application_controller.rb")
          insert_into_file "app/controllers/application_controller.rb", TXT, :after => "protect_from_forgery"
        else
          raise 'Couldn\'t find application_controller file'
        end
      end
    end
  end
end

config_generator_spec.rb

require 'spec_helper'
require 'generators/sth/config_generator'

def copy_files
  application_controller = File.expand_path("../templates/config/application_controller.rb", __FILE__)
  destination = File.join(destination_root, "app/controllers")

  FileUtils.mkdir_p(destination)
  FileUtils.cp application_controller, destination
end

describe Sth::Generators::ConfigGenerator do
  destination File.expand_path("../tmp", __FILE__)

  before do
    prepare_destination
    copy_files
  end

  it "configs the application controller" do
    run_generator
    f = file('app/controllers/application_controller.rb')
    f.should contain(/#{Sth::Generators::ConfigGenerator::TXT}/)
  end

  describe 'when application_controller.rb does not exist' do
    let(:real_application_controller_path ) { Rails.root + 'app/controllers/application_controller.rb'}
    let(:stash_application_controller_path) { 'stash_application_controller.rb'}
    before do
      FileUtils.mv real_application_controller_path, stash_application_controller_path
    end
    after do
      FileUtils.mv stash_application_controller_path, real_application_controller_path
    end
    it 'should raise an error' do
      expect { run_generator }.to raise_error "Couldn't find application_controller file"
    end
  end
end