Testing a Generator in a Rails Engine

Testing a generator in a Rails engine can result in unwanted modifications of files in the test/dummy directory.

# lib/generators/my_engine/install_generator.rb
module MyEngine
  module Generators
    class InstallGenerator < Rails::Generators::Base
      source_root File.expand_path("templates", __dir__)
      
      def create_users_table
        generate "migration", "create_users_table email:string:index password_digest:string"
      end
      
      def add_routes
        route "root to: 'home#index'"
      end      
    end
  end
end

What's Going On Here?

This class creates a new rails g my_engine:install command. When it is called a migration will be added to db/migrate/ and config/routes.rb will be updated.

This will happen even when the test suite is run and will affect the test/dummy directory resulting in unwanted changes.

# test/lib/generators/my_engine/install_generator_test.rb
require "test_helper"
require "generators/my_engine/install_generator"

class MyEngine::InstallGeneratorTest < Rails::Generators::TestCase
  tests ::MyEngine::Generators::InstallGenerator
  destination Rails.root

  setup :prepare_destination
  teardown :restore_destination

  test "creates migration for users table" do
    run_generator

    assert_migration "db/migrate/create_users_table.rb"
  end
  
  test "should add routes" do
    run_generator

    assert_file "config/routes.rb" do |file|
      assert_match(/root to: 'home#index'/, file)
    end
  end  

  private

  def backup_file(path)
    copy_file Rails.root.join(path), Rails.root.join("#{path}.bak")
  end

  def prepare_destination
    backup_file("config/routes.rb")
  end

  def remove_if_exists(path)
    full_path = Rails.root.join(path)
    FileUtils.rm_rf(full_path)
  end

  def restore_destination
    remove_if_exists("db/migrate")
    restore_file("config/routes.rb")
  end

  def restore_file(path)
    File.delete(Rails.root.join(path))
    copy_file Rails.root.join("#{path}.bak"), Rails.root.join(path)
    File.delete(Rails.root.join("#{path}.bak"))
  end
end

What's Going On Here?

We set the destination to the Rails.root. That way when run_generator is called the changes happen in the dummy app located in test/dummy.

We override the existing prepare_destination instance method with our own. This will be called before each test block is run, and serves to keep the test/dummy directory in a clean state. We call restore_destination to be called after each test is run via the teardown callback. This will restore any modified files and remove any generated files.

References

https://github.com/excid3/noticed/blob/master/test/generators/model_generator_test.rb

stevepolitodesign/example.md

Testing a Generator in a Rails Engine