missing_spec_generator.rb

require 'erb'

# lib/missing_spec_generator.rb
class MissingSpecGenerator
  RAILS_ROOT = File.expand_path(File.join(File.dirname(__FILE__), '..'))

  def spec_file spec_path, file_name, spec_template, namespace
    spec_name = file_name.gsub('.rb', '') + '_spec.rb'
    if File.exist?("#{spec_path}/#{spec_name}")
      puts "#{spec_path}/#{spec_name} exists"
    else
      puts "#{spec_path}/#{spec_name} missing"
      puts "\n"
      spec_file = ERB.new(spec_template)
      class_name = "#{namespace}#{file_name.gsub('.rb', '').camelcase}"
      spec = spec_file.result(binding)
      puts spec
      FileUtils.mkdir_p(spec_path) unless File.exist?(spec_path)
      File.open("#{spec_path}/#{spec_name}", 'w') {|f| f.write(spec) }
      puts "\n"
    end
  end

  def traverse_specs dir_name, spec_template, namespace=''
    source_path = RAILS_ROOT + '/app/' + dir_name
    unless File.directory?(source_path)
      puts "No #{dir_name}"
      return
    end
    Dir.open(source_path).each do |file_name|
      unless file_name.match(/^\./) # skip hidden folders (.svn)
        if File.directory?(RAILS_ROOT + '/app/' + dir_name + '/' + file_name)
          traverse_specs("#{dir_name}/#{file_name}", spec_template,
                         "#{namespace}#{file_name.camelcase}::")
        else
          spec_file("#{RAILS_ROOT}/spec/#{dir_name}", file_name, spec_template,
                    namespace)
        end
      end
    end
  end

  def generate_missing_helper_specs
    helper_template = %q{require 'rails_helper'

      RSpec.describe <%= class_name %>, type: :helper do
        pending "add some examples to (or delete) #{__FILE__}"
      end
      }.gsub(/^      /, '')
    traverse_specs('helpers', helper_template)
  end

  def generate_missing_controller_specs
    controller_template = %q{require 'rails_helper'

      RSpec.describe <%= class_name %>, type: :controller do
        pending "add some examples to (or delete) #{__FILE__}"
      end
      }.gsub(/^      /, '')
    traverse_specs('controllers', controller_template)
  end

  def generate_missing_worker_specs
    worker_template = %q{require 'rails_helper'

      RSpec.describe <%= class_name %>, type: :worker do
        pending "add some examples to (or delete) #{__FILE__}"
      end
      }.gsub(/^      /, '')
    traverse_specs('workers', worker_template)
  end

  def generate_missing_model_specs
    model_template = %q{require 'rails_helper'

      RSpec.describe <%= class_name %>, type: :model do
        pending "add some examples to (or delete) #{__FILE__}"
      end
      }.gsub(/^      /, '')
    traverse_specs('models', model_template)
  end
end

rake_task.rake

namespace :my_project do
  desc "Generate the missing rspec files from base templates"
  task :generate_missing_specs do
    require 'missing_spec_generator'
    msg = MissingSpecGenerator.new
    msg.generate_missing_helper_specs
    msg.generate_missing_controller_specs
    msg.generate_missing_model_specs
    msg.generate_missing_worker_specs
  end
end

Assuming this is rails lib, you could do like

RAILS_ROOT = Rails.root or even replace RAILS_ROOT with Rails.root