How to fix "NameError: uninitialized constant Thor::Base" error? - ruby-on-rails

Question

I am in the process of developing a Rails Engine (currently using Rails 4.1.6). I get this error when I try to run any Rake tasks:
$ bundle exec rake app:db:migrate --trace
** Invoke load_app (first_time)
** Execute load_app
rake aborted!
NameError: uninitialized constant Thor::Base
/Users/andrew/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/bundler-1.7.3/lib/bundler/ui/shell.rb:12:in `initialize'
/Users/andrew/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/bundler-1.7.3/lib/bundler/gem_helper.rb:26:in `new'
/Users/andrew/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/bundler-1.7.3/lib/bundler/gem_helper.rb:26:in `initialize'
/Users/andrew/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/bundler-1.7.3/lib/bundler/gem_helper.rb:13:in `new'
/Users/andrew/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/bundler-1.7.3/lib/bundler/gem_helper.rb:13:in `install_tasks'
/Users/andrew/example_rails_engine/Rakefile:10:in `<top (required)>'
/Users/andrew/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/rake-10.3.2/lib/rake/rake_module.rb:28:in `load'
/Users/andrew/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/rake-10.3.2/lib/rake/rake_module.rb:28:in `load_rakefile'
/Users/andrew/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/rake-10.3.2/lib/rake/application.rb:687:in `raw_load_rakefile'
/Users/andrew/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/rake-10.3.2/lib/rake/application.rb:94:in `block in load_rakefile'
/Users/andrew/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/rake-10.3.2/lib/rake/application.rb:176:in `standard_exception_handling'
/Users/andrew/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/rake-10.3.2/lib/rake/application.rb:93:in `load_rakefile'
/Users/andrew/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/rake-10.3.2/lib/rake/application.rb:77:in `block in run'
/Users/andrew/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/rake-10.3.2/lib/rake/application.rb:176:in `standard_exception_handling'
/Users/andrew/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/rake-10.3.2/lib/rake/application.rb:75:in `run'
/Users/andrew/.rbenv/versions/2.1.3/lib/ruby/gems/2.1.0/gems/rake-10.3.2/bin/rake:33:in `<top (required)>'
/Users/andrew/.rbenv/versions/2.1.3/bin/rake:23:in `load'
/Users/andrew/.rbenv/versions/2.1.3/bin/rake:23:in `<main>'
My Rakefile is as follows:
begin
require 'bundler/setup'
rescue LoadError
puts 'You must `gem install bundler` and `bundle install` to run rake tasks'
end
APP_RAKEFILE = File.expand_path("../spec/dummy/Rakefile", __FILE__)
load 'rails/tasks/engine.rake'
Bundler::GemHelper.install_tasks
This error doesn't make sense to me. I'm guessing Thor is a dependency of Rails Rake tasks. Where is this error coming from and how do I fix it?

Answer 1

adding
require 'thor'
to your RakeFile before
Bundler::GemHelper.install_tasks
fixed the issue for me

Answer 2

This seems to be an issue with either:
Bundler: https://github.com/bundler/bundler/issues/3205
or guard-rspec: https://github.com/guard/guard-rspec/issues/258
Guard lists Thor as a dependency, while Bundler vendors its own version of Thor, causing some sort of conflict.
I was able to temprorarily fix the problem by changing the order in which the code is loaded:
Move this line: Bundler::GemHelper.install_tasks above these lines:
APP_RAKEFILE = File.expand_path("../spec/dummy/Rakefile", __FILE__)
load 'rails/tasks/engine.rake'