By default, Rails doesn't add subfolders of the models directory to the autoload path. Which is why it can only find namespaced models -- the namespace illuminates the subdirectory to look in.
To add all subfolders of app/models to the autoload path, add the following to config/application.rb:
config.autoload_paths += Dir[Rails.root.join("app", "models", "{*/}")]
Or, if you have a more complex app/models directory, the above method of globing together all subfolders of app/models may not work properly. In which case, you can get around this by being a little more explicit and only adding the subfolders that you specify:
config.autoload_paths += Rails.root.join("app", "models", "<my_subfolder_name1>")
config.autoload_paths += Rails.root.join("app", "models", "<my_subfolder_name2>")
UPDATE for Rails 4.1+
As of Rails 4.1, the app generator doesn't include config.autoload_paths
by default. So, note that the above really does belong in config/application.rb.
UPDATE
Fixed autoload path examples in the above code to use {*/}
instead of {**}
. Be sure to read muichkine's comment for details on this.