Following my series on job interviews which started here Job Interview - Rails Main Components This post will have new questions. Hopefully some people will learn a few things, at least I did. Without further ado the questions and answers; enjoy!.

Name at least 3 ways to call a method in Ruby?

 class Greeter
    def greeting

greeter = 

# First way and most obvious 

# Second way not so obvious but probably well known 
greeter.send :greeting 

# Third way the more obscure 

What is the difference between a Class, Module and Instance?

  • Class: is the blueprint from which individual objects are created. Classes in Ruby are first-class objects—each is an instance of class Class.
  • Instance: is an object which was instantiated from a Class
  • Module: is a collection of methods and constants. You can’t make an instance of a module; the way you access the constants and methods inside it depends on it’s definition; module methods and variables can be accessed like this Module.method or Module.CONSTANT but if you want to access an instance method you should include the module to another class to use it.

What is the difference between include and extend?

include mixes a module as instance methods or constants and extend mixes a module as class methods; also you can extend a singleton object instance using this syntax. obj.extend Mod which will add the methods or constants to that single instance of that particular class.

What is a symbol and what are the risks of using them in web apps?

A symbol is mostly an immutable String they are mostly use to represent names. They are risky to be use on web apps in particular in Rails apps due to the following security issue:CVE-2013-1854 Symbol DoS vulnerability in Active Record

Update: This cannot lead an attacker to execute arbitrary code in the server you are running your app. That was a misinterpretation on my part. Thanks to Toby Ovod-Everett for pointing this out.

Can you name a recent security incident in Rails and explain the issue?

This was not very easy for me; because I normally don’t spend too much time on this type of stuff this days; even when I know the important of the topic; so the answer for this one is just to subscribe to the rails-security-ann mailing list

What does this do Hash[[1,2,3,4].zip([5,6,7,8])]?

Enumerable#zip Takes one element from enum and merges corresponding elements from each args.

  {1=>5, 2=>6, 3=>7, 4=>8}

What does the following do

(1..Float::INFINITY).map{ |i| i*i }.first(10)?

Nothing it will just hang your Ruby interpreter.

How to fix this

  (1..Float::INFINITY).map{ |i| i*i }.first(10)?

This only works on Ruby 2.0

(1..Float::INFINITY){ |i| i*i }.first(10)

What is the difference between a Proc and Lambda?

  • Block: Is a piece of code that can pass to a method as an argument. But can’t save it’s own state.

  • Proc: Is a block which can save state.

  • Lambda: Same thing as a Proc; the differences are that the lambda have diminutive returns; which means that even if you put a return statement inside the lambda it will keep running until the method in which was called finish it’s execution also lambdas check for the arguments passed to them; Proc doesn’t.


I hope this help anyone interesting in learning tip bits of Ruby. Also I also would love to see another methods to solve the problems exposed in this questions. Happy coding!


Ruby Object

What is a Class?

Ruby Module

Ruby Class

Include and Extend



Blocks, Procs, Lambdas