In what order are the config files loaded for Rails?

Question

I'm in the process of trying to initialize different gems for different environments. I'm using initializer config files (for things like Paperclip) and environment config files (for my dev, test, qa, prod environments).

For some context, I'm trying to get my prod and qa servers to use S3 storage for Paperclip, but use local storage with different directories for dev and test. I have no idea in what order these config files are loaded.

I was wondering if someone could shed some light on the load order so that I can make sure I've got any dependencies or overrides correct. Also, I just like to know how these things work.

I'm particularly interested in the directories/files listed below

config/     environments/         develop.rb         test.rb         ...env-specific config files       initializers/         paperclip.rb         ...gem-specific config files      application.rb     boot.rb     deploy.rb     environment.rb     routes.rb 

Thanks!

Answer 1

Rails 6.0

Updated Nov 2019: Initialization Process and Configuration

Same as Rails 5.2

Rails 5.2

Updated Nov 2019:Initialization Process Configuration

Launch

1. railties/exe/rails
2. railties/lib/rails/app_loader.rb
3. bin/rails
4. config/boot.rb
5. rails/commands.rb
6. rails/command.rb
7. actionpack/lib/action_dispatch.rb
8. rails/commands/server/server_command.rb
9. Rack: lib/rack/server.rb
10. config/application
11. Rails::Server#start
12. config/environment.rb
13. config/application.rb

Load

1. railties/lib/rails/all.rb
2. Back to config/environment.rb
3. railties/lib/rails/application.rb
4. Rack: lib/rack/server.rb

Rails 4.2

Updated Sep 2013: For Rails 4 it appears to have changed again. There is now a Rails-4 Guide on The Rails Initialization Process. You'll notice that this list is much shorter than the one for Rails 3. I'm not sure if they removed some of the depth or what... Haven't had the time to go over it all:

1. bin/rails
2. config/boot.rb
3. rails/commands.rb
4. actionpack/lib/action_dispatch.rb
5. rails/commands/server.rb
6. Rack: lib/rack/server.rb
7. config/application
8. Rails::Server#start
9. config/environment.rb
10. config/application.rb
11. railties/lib/rails/all.rb
12. Back to config/environment.rb
13. railties/lib/rails/application.rb
14. Rack: lib/rack/server.rb

For more detailed information on how to configure some of these files see the Rails-4 Guide on Configuring Rails Applications

---

Rails 3.2

Updated Sep 2013: For Rails 3 it appears to have changed a lot. There is now a Rails-3 Guide on The Rails Initialization Process:

1. bin/rails railties/lib/rails/cli.rb script/rails config/boot.rb
2. rails/commands.rb actionpack/lib/action_dispatch.rb
3. activesupport/lib/active_support.rb
4. activesupport/lib/active_support/lazy_load_hooks.rb
5. activesupport/lib/active_support/inflector/methods.rb
6. actionpack/lib/action_dispatch.rb cont’d. rails/commands/server.rb
7. Rack: lib/rack/server.rb Rails::Server#start config/environment.rb
8. config/application.rb Loading Rails railties/lib/rails/all.rb
9. railties/lib/rails.rb railties/lib/rails/ruby_version_check.rb
10. active_support/core_ext/kernel/reporting.rb
11. active_support/core_ext/logger.rb railties/lib/rails/application.rb
12. active_support/file_update_checker.rb railties/lib/rails/plugin.rb
13. railties/lib/rails/engine.rb railties/lib/rails/railtie.rb
14. railties/lib/rails/initializable.rb
15. railties/lib/rails/configuration.rb
16. activesupport/lib/active_support/deprecation.rb
17. activesupport/lib/active_support/deprecation/behaviors.rb
18. activesupport/lib/active_support/notifications.rb
19. activesupport/core_ext/array/wrap
20. activesupport/lib/active_support/deprecation/reporting.rb
21. activesupport/lib/active_support/deprecation/method_wrappers.rb
22. activesupport/lib/active_support/deprecation/proxy_wrappers.rb
23. active_support/ordered_options railties/lib/rails/paths.rb
24. railties/lib/rails/rack.rb
25. activesupport/lib/active_support/inflector.rb
26. active_support/inflections
27. activesupport/lib/active_support/inflector/transliterate.rb Back to
28. railties/lib/rails/railtie.rb railties/lib/rails/engine/railties.rb
29. Back to railties/lib/rails/engine.rb Back to
30. railties/lib/rails/plugin.rb Back to
31. railties/lib/rails/application.rb railties/lib/rails/version.rb
32. activesupport/lib/active_support/railtie.rb
33. activesupport/lib/active_support/i18n_railtie.rb
34. railties/lib/rails/railtie/configuration.rb Back to
35. activesupport/lib/active_support/i18n_railtie.rb Back to
36. activesupport/lib/active_support/railtie.rb
37. activesupport/lib/action_dispatch/railtie.rb
38. activesupport/lib/action_dispatch.rb activemodel/lib/active_model.rb
39. activesupport/lib/active_support/i18n.rb Back to
40. activesupport/lib/action_dispatch.rb Back to
41. activesupport/lib/action_dispatch/railtie.rb Back to
42. railties/lib/rails.rb Back to railties/lib/rails/all.rb
43. activerecord/lib/active_record/railtie.rb
44. activerecord/lib/active_record.rb Back to
45. activerecord/lib/active_record/railtie.rb
46. actionpack/lib/action_controller/railtie.rb
47. actionpack/lib/action_view.rb

For more detailed information on how to configure some of these files see the Rails-3 Guide on Configuring Rails Applications

---

Rails 2.3

Originally (Dec 2011), I stumbled across a blog post that had an awesome explanation of How the Initialization Process Worked for Rails 2.

1. config/preinitializer.rb
2. config/environment.rb
3. config/environments/#{RAILS_ENV}.rb
4. plugin initialization
5. gem initialization
6. config/initializer/*.rb
7. all after_initialize blocks, in the order they were defined in (so same order as above)
8. any junk left below the Rails::Initializer.run call/block in environment.rb

For more detailed information on how to configure some of these files see the Rails-2 Guide on Configuring Rails Applications