dummy_generator.rb

require 'rails/generators'
require 'rails/generators/rails/plugin_new/plugin_new_generator'

module MyEngine
  class DummyGenerator < Rails::Generators::PluginNewGenerator

    def self.default_source_root
      Rails::Generators::PluginNewGenerator.default_source_root
    end

    def do_nothing
    end

    alias :create_root :do_nothing
    alias :create_root_files :do_nothing
    alias :create_app_files :do_nothing
    alias :create_config_files :do_nothing
    alias :create_lib_files :do_nothing
    alias :create_public_stylesheets_files :do_nothing
    alias :create_javascript_files :do_nothing
    alias :create_script_files :do_nothing
    alias :update_gemfile :do_nothing
    alias :create_test_files :do_nothing
    alias :finish_template :do_nothing

  end
end

Rakefile

#!/usr/bin/env rake

require 'rspec/core/rake_task'
RSpec::Core::RakeTask.new(:spec => :dummy_app) do |t|
  t.pattern =  File.expand_path('../spec/**/*_spec.rb', __FILE__)
end
task :default => :spec

desc 'Generates a dummy app for testing'
task :dummy_app => [:setup, :migrate]

task :setup do
  require 'rails'
  require 'my_engine'
  require 'my_engine/generators/dummy_generator'

  dummy = File.expand_path('../spec/dummy', __FILE__)
  sh "rm -rf #{dummy}"
  MyEngine::DummyGenerator.start(
    %W(. --quiet --force --skip-bundle --old-style-hash --dummy-path=#{dummy})
  )
end

task :migrate do
  rakefile = File.expand_path('../spec/dummy/Rakefile', __FILE__)
  sh("rake -f #{rakefile} my_engine:install:migrations")
  sh("rake -f #{rakefile} db:create db:migrate db:test:prepare")
end