Why does Rails give “`require': cannot load such file — singleton” only on production server?

Question

I'm trying to convert a Rails 4.0 project from PostgreSQL to MySQL.

I have everything working in my local Vagrant VM, but after I do cap deploy , this is what I see in the logs on the production server:

I, [2015-03-25T15:39:12.558662 #28566]  INFO -- : Refreshing Gem list
[snip]/shared/bundle/ruby/2.2.0/gems/activesupport-4.0.13/lib/active_support/deprecation.rb:1:in `require': cannot load such file -- singleton (LoadError)
        from [snip]/shared/bundle/ruby/2.2.0/gems/activesupport-4.0.13/lib/active_support/deprecation.rb:1:in `<top (required)>'
        from [snip]/shared/bundle/ruby/2.2.0/gems/railties-4.0.13/lib/rails/configuration.rb:1:in `require'
        from [snip]/shared/bundle/ruby/2.2.0/gems/railties-4.0.13/lib/rails/configuration.rb:1:in `<top (required)>'
        from [snip]/shared/bundle/ruby/2.2.0/gems/railties-4.0.13/lib/rails/railtie.rb:2:in `require'
        from [snip]/shared/bundle/ruby/2.2.0/gems/railties-4.0.13/lib/rails/railtie.rb:2:in `<top (required)>'
        from [snip]/shared/bundle/ruby/2.2.0/gems/railties-4.0.13/lib/rails/engine.rb:1:in `require'
        from [snip]/shared/bundle/ruby/2.2.0/gems/railties-4.0.13/lib/rails/engine.rb:1:in `<top (required)>'
        from [snip]/shared/bundle/ruby/2.2.0/gems/railties-4.0.13/lib/rails/application.rb:4:in `require'
        from [snip]/shared/bundle/ruby/2.2.0/gems/railties-4.0.13/lib/rails/application.rb:4:in `<top (required)>'
        from [snip]/shared/bundle/ruby/2.2.0/gems/railties-4.0.13/lib/rails.rb:9:in `require'
        from [snip]/shared/bundle/ruby/2.2.0/gems/railties-4.0.13/lib/rails.rb:9:in `<top (required)>'
        from [snip]/shared/bundle/ruby/2.2.0/gems/railties-4.0.13/lib/rails/all.rb:1:in `require'
        from [snip]/shared/bundle/ruby/2.2.0/gems/railties-4.0.13/lib/rails/all.rb:1:in `<top (required)>'
        from [snip]/releases/20150325221423/config/application.rb:3:in `require'
        from [snip]/releases/20150325221423/config/application.rb:3:in `<top (required)>'
        from [snip]/releases/20150325221423/config/environment.rb:2:in `require'
        from [snip]/releases/20150325221423/config/environment.rb:2:in `<top (required)>'
        from config.ru:4:in `require'
        from config.ru:4:in `block in <main>'
        from [snip]/shared/bundle/ruby/2.2.0/gems/rack-1.5.2/lib/rack/builder.rb:55:in `instance_eval'
        from [snip]/shared/bundle/ruby/2.2.0/gems/rack-1.5.2/lib/rack/builder.rb:55:in `initialize'
        from config.ru:1:in `new'
        from config.ru:1:in `<main>'
        from [snip]/shared/bundle/ruby/2.2.0/gems/unicorn-4.8.3/lib/unicorn.rb:48:in `eval'
        from [snip]/shared/bundle/ruby/2.2.0/gems/unicorn-4.8.3/lib/unicorn.rb:48:in `block in builder'
        from [snip]/shared/bundle/ruby/2.2.0/gems/unicorn-4.8.3/lib/unicorn/http_server.rb:764:in `call'
        from [snip]/shared/bundle/ruby/2.2.0/gems/unicorn-4.8.3/lib/unicorn/http_server.rb:764:in `build_app!'
        from [snip]/shared/bundle/ruby/2.2.0/gems/unicorn-4.8.3/lib/unicorn/http_server.rb:628:in `init_worker_process'
        from [snip]/shared/bundle/ruby/2.2.0/gems/unicorn-4.8.3/lib/unicorn/http_server.rb:651:in `worker_loop'
        from [snip]/shared/bundle/ruby/2.2.0/gems/unicorn-4.8.3/lib/unicorn/http_server.rb:525:in `spawn_missing_workers'
        from [snip]/shared/bundle/ruby/2.2.0/gems/unicorn-4.8.3/lib/unicorn/http_server.rb:536:in `maintain_worker_count'
        from [snip]/shared/bundle/ruby/2.2.0/gems/unicorn-4.8.3/lib/unicorn/http_server.rb:294:in `join'
        from [snip]/shared/bundle/ruby/2.2.0/gems/unicorn-4.8.3/bin/unicorn:126:in `<top (required)>'
        from [snip]/shared/bundle/ruby/2.2.0/bin/unicorn:23:in `load'
        from [snip]/shared/bundle/ruby/2.2.0/bin/unicorn:23:in `<main>'
E, [2015-03-25T15:39:12.723362 #1450] ERROR -- : reaped #<Process::Status: pid 28563 exit 1> worker=0

But my understanding is that singleton is a Ruby feature, not some third-party library that I might be missing. Is that correct? What is the problem here?

Edit: I am in the process of adding Chef to this project. I have set up a second Vagrant VM to test the production config on before I apply it to the actual production server. I have configured this VM to match the production server, as close as I can tell, but the VM works fine.

Answer 1

I finished adding Chef to the project and now use it to configure the production server. Around the same time I also upgraded to Rails 4.2.

I'm no longer getting this error, so it is likely one of those two things that fixed the problem.