Ruby Forensics

This guest post is contributed by Elise Huard, who is based in Brussels, Belgium and is the owner of Jabberwocky, a solutions company mostly focused on Rails. She has worked with a few other technologies before falling in love with Rails and Ruby about 3 years ago and going freelance to work with Ruby full time. She contributes to open source projects as much as she can, and has given talks at a few Ruby and Rails conferences. She’s a jack of all trades, loves reading, tinkering, food, travel, learning, and people out of the ordinary.

Elise Huard

Ruby Forensics

Say you want to use a library, but no or very little documentation is available, and you don’t feel like diving into the code right away.

Well, you picked the right language. Ruby is blessed with what is called introspection: if you ask a Ruby program/class/module politely, it will tell you almost anything about itself. This post will tell you some tricks I use on a daily basis.

module Layer
  FILLINGS = [:chocolate, :meringue, :jam, :cream, :strawberry]
  def fill(filling)
    puts "fill with #{filling}"

class Cake
  include Layer
  attr_accessor :calories

  def ice
    @calories = @calories + 200

  def eat
    puts "nom"

  def self.bake
    return new

class CheeseCake < Cake; end

Say you have a class, but you’d like to know what method it defines.


Won’t be that useful, because Ruby objects (with some exceptions like BasicObject) have got a whole lot of methods out of the box. Rather, use:

Cake.public_instance_methods - Object.public_instance_methods
=> [:calories, :calories=, :ice, :eat, :fill]

If you want one particular method, and you have an idea of which name it should have:

(Cake.public_instance_methods - Object.public_instance_methods).grep(/eat/)

Will show if your instinct was right or not.

The same can be done for class methods, of course:

(Cake.methods - Object.methods)
=> [:bake]

(public_class_methods also works.)

Note: this won’t show you the dynamic methods, like find_by_X for ActiveRecord. The class doesn’t know it has these kind of methods in itself. They’re executed on the fly when the program hits method_missing. You’d have to look at the classes’ method_missing method to find out.

Then there’s the case where you’d like to know, exactly, where a method was defined. Ruby gives us the method method, which takes a symbol as an argument.
 => #<Method: Cake(Layer)#fill>

Which tells us it was defined in the Layer module, included in the Cake class.
 => #<Method: Cake#eat>

Sometimes, you want to know which other classes your class was descended from – including mixed in modules. The ancestors method will show you:

 => [CheeseCake, Cake, Layer, Object, Kernel, BasicObject]

Should you want to know about any constants, there’s a method for that too:


All these methods have a good chance of giving you the information you want. When used in irb, together with a little experimentation, they can really help you find the code you’re looking for.

I hope you found this article valuable. Feel free to ask questions and give feedback in the comments section of this post. Thanks!

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