"class declarations are loaded as constants at run time" Let's say I create a class called User. When ruby sees that declaration, it creates a constant called

"class declarations are loaded as constants at run time" Let's say I create a class called User. When ruby sees that declaration, it creates a constant called User to contain that object. When I create a new instance: user = User.new "User" is a constant whose value is the object/class. "new" is a method that calls the constructor (if one exists) and creates a new instance. The same applies when you call something like: User.find You're not calling find() on an instance, you're calling it on the object that's loaded into the constant "User". The class declaration "class User" is saying User is the name of the class; User is also the constant in which it's stored. It's also what allows you to reopen a class on the fly to extend it with more methods. "I think there's a third purpose to DI that you didn't mention. The container does instantiate an object and inject it ("wire it") into another object, but the real benefit is that you can externally configure which object to swap in." True, but I've never found myself switching objects out on a regular basis. As you mentioned, the conditional approach probably isn't the best. What I'm assuming now is that you're based on your deployment, you might use a different bean factory configuration file? And this works as long as UserLdap and UserDb implement the same interface? That being said, instead of sprinkling conditionals throughout user, you can dynamically assign User initially. User = UserDb OR User = UserLdap and as long as UserDb or UserLdap had a find() method, you could then call User.find Remember, User is now a constant that references the same value as UserDb or UserLdap Does that help clarify?