Understanding class variables and methods in Ruby on Rails - ruby-on-rails

Question

I'm new to Ruby and Ruby on Rails, coming from a background of C-like languages.
Here is some code that I found in the application_controller.rb file:
def current_user
#current_user ||= Renter.find(session[:user_id]) if session[:user_id]
end
helper_method :current_user
def authorize_user
redirect_to '/login' unless current_user
end
Here is what I don't understand about it:
- On line 4, is :current_user invoking the current_user instance method, or directly accessing the #current_user instance variable?
- On line 7, is current_user invoking the current_user instance method, or directly accessing the #current_user instance variable?
- On line 2, is :user_id a variable or is it more like a string literal being used as a key? Kind of like in JavaScript one might write session["user_id"] to get theuser_id property of the session object.

Answer 1

class methods aren't relevant in this example - they aren't being used here.
Instance variables will never call methods when they are get/set.
Although the opposite does sometimes happen. It's a very common pattern to create getter/setter methods for instance variables, so common that attr reader/writer/accessor helpers are defined in ruby core. If you write attr_accessor :foo, then foo= and foo will get/set the instance variable.
But this does not happen by default.
To answer your other question:
A symbol :user_id starts with a colon and is similar to a string. The difference between a symbol and a string may seem arbitrary, but it is an important concept in Ruby and making the distinction in your head is a good idea.
To respond to your comment:
Line 4, helper_method :current_user is really something specific to rails, consider it "rails magic" if you like. In effect this is making your current_user method callable from views (whereas by default it would only be available in the controller). :current_user is a symbol which is used to reference the current_user method. Not necessarily something you have to understand in total detail, it would suffice to know that helper_method takes a symbol with the same name as a method and makes that method available to views. As far as I'm aware, it's only relevant to Rails controllers.
It's somewhat common in Ruby to use symbols that refer to method names. It's a more intermediate concept. You can see another example in send:
def asd
return 0
end
class Foo
def instance_method_bar
return 0
end
def self.class_method_bar
return 0
end
end
# how the methods are typically called
asd
Foo.new.instance_method_bar
Foo.class_method_bar
# another way to call them, using send
send(:asd)
Foo.new.send(:instance_method_bar)
Foo.send(:class_method_bar)
I'm not recommending you use send unless you need to, but hopefully it will make it more clear how the symbol :current_user is being used in helper_method
Line 7 is the current_user method being called.

Answer 2

Let's tackle your questions one at a time.
On line 4, :current_user is a method, most likely used to return the current_user, so you can access username, or email, or whatever value the user has.
On line 7, it is still the same method. In this case, Ruby is checking whether a current_user object exists. You can think of unless as if not. So the code will redirect to login if current_user is false, which will happen if current_user == nil. If the user is logged in, current_user != nil, and the redirect does not happen.
On line 2, :user_id is a symbol, and session is a hash, which is key-value pair, such as { a: 1, b: 2 }, and you access the value with the key using the [] method, so session[:user_id] is returning the value of the user_id. In Ruby, you can use anything as the key, symbols are used because they are always unique, the object id of :two and :two is the same, whereas the id is different for "two" and "two".