How to regenerate config/application.rb in Rails 4.2?

Question

Does Rails 4.2 provide a single command that regenerates config/application.rb in an existing application?

The reason I ask is imagine a Rails app is rails new-ed without the --skip-test-unit option.

Then at a much later date, this app is switched to RSpec. How can config/application.rb be regenerated as if the --skip-test-unit option had been supplied originally to rails new ...?

All this would effectively do is change the require statements near the top of config/application.rb file from:

require 'rails/all'

to:

require "rails"
# Pick the frameworks you want:
require "active_model/railtie"
require "active_job/railtie"
require "active_record/railtie"
require "action_controller/railtie"
require "action_mailer/railtie"
require "action_view/railtie"
require "sprockets/railtie"
# require "rails/test_unit/railtie"

Answer 1

Delete the existing config/application.rb and, in the directory above the project root, run rails new ... with the options you want, including the application directory name, and the --skip-test-unit option, and the --skip option, which makes rails new ... skip creating files that already exist:

# Inside your-rails-app/ directory:

# 1. Remove config/application.rb
rm config/application.rb

# 2. Move up one directory
cd ..

# 3. Regenerate rails application with the --skip
#    option, using the name of your-rails-app. The
#    --skip option will not generate files that
#    already exist.
#     
#    This assumes you have rails installed as a global
#    gem, of the version given in the command (version
#    x.y.z in this case, replace with the version of Rails
#    your app uses, e.g. 4.2.88):
rails _x.y.z_ new your-rails-app --skip --skip-test-unit

# 4. Run git status to see if any files were unexpectedly added
#    (for example the README.rdoc file will be created
#    if it did not exist).
git status

# 5. Clean up and commit the changes you want.

And that's it!