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I'm trying to create a non-ActiveRecord model in app/models/gamestate.rb. Then inside my controller (PlayController) I should be able to do GameState.new, right? No go:

NameError (uninitialized constant PlayController::GameState):
  app/controllers/play_controller.rb:23:in `play'

(at least in the development environment)

But! If I do have a model called app/models/play.rb, then it's automatically loaded and I can do Play.new.

So my question is: how does Rails know which classes to load? What sort of name mangling does it do to get from play#action to PlayController to app/controllers/play_controller.rb to app/models/play.rb?

It seems awfully fragile, but maybe a better understanding of how this works would help.

And finally, how can I get it to load app/models/gamestate.rb?

PBJ
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  • Wow --- naming it `app/models/game_state.rb` fixes the problem, after hours of Googling and reading a hint here: http://stackoverflow.com/questions/4074830/adding-lib-to-config-autoload-paths-in-rails-3-does-not-autoload-my-module My question still stands, though: I'd like to get a better understanding of how this name change magic works. Specifically, I'd like to know which aspects of these naming conventions are simply convention, and which are really required. And do these conventions come from Ruby, or Rails? – PBJ Dec 26 '10 at 09:54

3 Answers3

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Heikki is right, but doesn't explain why it's this way. Rails adds every folder inside app to the autoload_path config setting. When an unknown constant is asked for, Rails will look up in this path to find the constant and if it can't find a filename -- such as the case of gamestate.rb vs. the proper game_state.rb -- then it won't be able to load the constant defined in this file.

Ryan Bigg
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  • Thanks -- "how it works" is exactly what I was looking for. So does the routing system simply capitalize the controller and append to it, like "game" => "GameController"? Where is the logic for this implemented? – PBJ Dec 26 '10 at 21:08
  • @David: Yes, to my knowledge the routing system uses the reverse, where it would take game_states#index and match it to the GameStatesController class. – Ryan Bigg Dec 26 '10 at 21:28
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To expand on Ryan's answer, this is a Rails convention. In Rails 3, activesupport/lib/dependencies.rb has a method called load_missing_constant that would look for GameState when you reference it and it hasn't already been loaded. It takes the missing constant 'GameState', calls 'underscore' on it which converts it to game_state, and then searches for it in the autoload_paths. If it finds a file with that name (ending in .rb), it loads it.

In production mode, game_state.rb would be loaded up front (assuming it's in one of the autoload_paths) so the GameState class constant would already be loaded by the time you reference it. No mangling needed in that case.

You might want to put non-AR models in a separate folder; for example you can add to the autoload paths in config/application.rb, like:

config.autoload_paths += %W(#{config.root}/lib)

Then you could put class GameState in lib/game_state.rb and Rails would find it.

BTW, I'm not a Rails expert, but one way to figure this stuff out is by running with the Ruby debugger and setting breakpoints where 'magic' seems to happen; then you can use where or caller(0) to backtrack and find the relevant Rails code. Sometimes it takes a few tries with strategic breakpoints (or by adding debugger to the source code), but it usually makes sense once you see it happen line by line.

Giacomo1968
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Brian Deterling
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  • Also see the next answer (by Ryan) where he explains how rails auto loading works, and why, in this case, using the filename of `gamestate.rb` instead of `game_state.rb` breaks rails' autoloading. – kapad Jul 17 '18 at 21:54
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If the model filename is gamestate.rb then the class name should be Gamestate instead of GameState (alternatively game_state.rb -> GameState). Thats Rails naming convention. I recommend following them where possible. Things go much more fluent that way.

Article about Ruby/Rails naming conventions: http://itsignals.cascadia.com.au/?p=7

Heikki
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