Understanding class variables and methods in Ruby on Rails -

i'm new ruby , ruby on rails, coming background of c-like languages.
here code found in 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 don't understand it:
- on line 4, :current_user invoking current_user instance method, or directly accessing @current_user instance variable?
- on line 7, current_user invoking current_user instance method, or directly accessing @current_user instance variable?
- on line 2, :user_id variable or more string literal being used key? kind of in javascript 1 might write session["user_id"] theuser_id property of session object.

class methods aren't relevant in example - aren't being used here.

instance variables never call methods when get/set.

although opposite happen. it's common pattern create getter/setter methods instance variables, common attr reader/writer/accessor helpers defined in ruby core. if write attr_accessor :foo, foo= , foo get/set instance variable.

but not happen default.

to answer other question:

a symbol :user_id starts colon , similar string. difference between symbol , string may seem arbitrary, important concept in ruby , making distinction in head idea.

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to respond comment:

line 4, helper_method :current_user specific rails, consider "rails magic" if like. in effect making current_user method callable views (whereas default available in controller). :current_user symbol used reference current_user method. not have understand in total detail, suffice know helper_method takes symbol same name method , makes method available views. far i'm aware, it's relevant rails controllers.

it's common in ruby use symbols refer method names. it's more intermediate concept. can see 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 methods typically called asd foo.new.instance_method_bar foo.class_method_bar  # way call them, using send send(:asd) foo.new.send(:instance_method_bar) foo.send(:class_method_bar) 

i'm not recommending use send unless need to, make more clear how symbol :current_user being used in helper_method

line 7 current_user method being called.