Caching With Instance Variables

class ApplicationController < ActionController::Base
    def current_user
        @current_user ||= User.find(session[:user_id])
    end
end

this piece of codes are much better than the following pieces:

class ApplicationController < ActionController::Base
    def current_user
        User.find(session[:user_id])
    end
end

Because in the first part, the searching result is saved in the instance variable, next time when get the current user, it will not execute the search again, it just return that variable.

But in the second piece of code, it will excute the search everytime you get the current user.

this can improve the performance.