🔍 Understanding Ruby’s Method Lookup Algorithm: A Must for Every Developer

June 12, 2025

If you’ve spent some time working with Ruby, you’ve probably run into situations where a method is being called… but you’re not quite sure where it’s defined. Is it in the class itself? A superclass? An included module?

Behind this magic is one of Ruby’s core mechanisms: the method lookup algorithm.

Understanding how Ruby finds and dispatches method calls isn’t just interesting—it’s essential. Whether you’re debugging, building mixins, or designing object-oriented architectures, knowing how Ruby walks the inheritance chain will help you write clearer and more predictable code.

🧠 What Happens When You Call a Method?

When you send a message (i.e., call a method) on an object in Ruby, here’s what happens:

1. Ruby starts with the object’s class.
2. It looks in that class’s method table.
3. If the method isn’t there, it follows the superclass pointer.
4. If a module was included, Ruby places it before the superclass in the lookup chain.
5. Ruby continues climbing the chain—modules, superclasses, all the way up to BasicObject—until it finds the method.

It’s a linear search, and it’s elegantly simple.

💡 Example: Classes, Modules, and Superclasses

Let’s break it down with a simple example:

module Professor
  def lectures
    "Gives lectures"
  end
end

class Person
  attr_accessor :first_name, :last_name
end

class Mathematician < Person
  include Professor
end

ramanujan = Mathematician.new
ramanujan.first_name = "Srinivasa"

When we assign first_name, Ruby looks for the method first_name=.

Here’s the path Ruby follows:

• Looks in Mathematician → method not found.
• Sees that Professor was included → looks there → method still not found.
• Moves to the original superclass, Person → method found!

You can see this hierarchy directly in code:

Mathematician.ancestors
# => [Mathematician, Professor, Person, Object, Kernel, BasicObject]

🔄 Why Does This Matter?

• Debugging: Knowing the lookup path can save hours of head-scratching when methods behave unexpectedly.
• Designing modules: Inclusion order matters. Modules can override superclass methods.
• Method overrides and super: You’ll know exactly what super is calling and why.

You can even use this to see where a method lives:

ramanujan.method(:first_name=).owner
# => Person

🧰 Final Thought

Ruby’s method lookup algorithm is one of the many things that makes the language feel intuitive yet powerful. It hides complexity behind a clean interface—but that simplicity is built on an elegant internal model worth understanding.

If you love Ruby as much as I do, dig into the internals once in a while. It will deepen your appreciation—and sharpen your skills.