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Method visibility in Ruby (public, protected, and private methods) has been well explained in places like this blog post. But in Ruby on Rails it seems slightly different than it would be in a regular Ruby application because of the way the framework is set up. So, in Rails models, controllers, helpers, tests, etc., when is/isn't it appropriate to use protected or private methods?
Edit: Thanks for the answers so far. I understand the concept of protected and private in Ruby, but I'm looking more for an explanation of the typical way those types of visibility are used within the context of the various pieces of a Rails app (models, controllers, helpers, tests). For example, public controller methods are action methods, protected methods in the application controller are used for "helper methods" that need to be accessed by multiple controllers, etc.
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Both protected and private methods cannot be called from the outside of the defining class. Protected methods are accessible from the subclass and private methods are not. Private methods of the defining class can be invoked by any instance of that class. Public access is the default one.
Protected methods are a balance between public and private methods. They are similar to private methods in that they cannot be accessed in the public scope. Neither the client nor the program can invoke them. However, objects of the same class can access each other's protected methods.
What is a private method in Ruby? It's a type of method that you can ONLY call from inside the class where it's defined. This allows you to control access to your methods.
protected methods can be called by any instance of the defining class or its subclasses. private methods can be called only from within the calling object. You cannot access another instance's private methods directly.
For models, the idea is that the public methods are the public interface of the class. Public methods are intended to be used by other objects, while protected/private methods are to be hidden from the outside.
This is the same practice as in other object-oriented languages.
For controllers and tests, just do as you please. Both controller and test classes are only instantiated and called by the framework (yes, I know you can theoretically get the controller from the view, but if you do that, something is strange anyway). Since no one will ever create those things directly, there's nothing to "protect" against.
Addendum/Correction: For controllers, you should mark the "helper" methods as protected private, and only the actions themselves should be public. The framework will never route any incoming HTTP calls to actions/methods that are not public, so your helper methods should be protected in that way.
For helpers it will make no difference if a method is protected or private, since they are always called "directly".
You can mark stuff protected in all those cases if it makes things easier for you to understand, of course.
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