Method lookup chain in Ruby

Introduction

The method lookup chain shows how Ruby finds the code to run when you call a method on an object. It helps understand which method runs if many have the same name.

You want to know which method will run when you call a method on an object.

You have multiple classes and modules with methods of the same name and want to see the order Ruby checks them.

You want to debug why a method behaves differently than expected.

You want to understand how Ruby handles inheritance and modules mixed into classes.

Syntax

Ruby

object.method(:method_name).owner

This code shows which class or module owns the method Ruby will run.

You can also use Module#ancestors to see the full lookup chain.

Examples

This shows that the hello method called on b is owned by class B, because B overrides A's method.

Ruby

class A
  def hello
    puts 'Hello from A'
  end
end

class B < A
  def hello
    puts 'Hello from B'
  end
end

b = B.new
p b.method(:hello).owner

This shows that the greet method comes from module M included in class C.

Ruby

module M
  def greet
    puts 'Hi from M'
  end
end

class C
  include M
end

c = C.new
p c.method(:greet).owner

This shows that to_s is owned by Ruby's Object class, because D does not define it.

Ruby

class D
end

d = D.new
p d.method(:to_s).owner

Sample Program

This program shows which speak method runs when called on child. It also prints the owner of the method and the full method lookup chain for class Child.

Ruby

module M1
  def speak
    puts 'Speak from M1'
  end
end

module M2
  def speak
    puts 'Speak from M2'
  end
end

class Parent
  def speak
    puts 'Speak from Parent'
  end
end

class Child < Parent
  include M1
  include M2
end

child = Child.new
child.speak
p child.method(:speak).owner
p Child.ancestors

Important Notes

The method lookup chain starts from the object's class, then checks included modules in reverse order of inclusion, then parent classes.

Modules included later override methods from modules included earlier.

You can use ancestors to see the full chain Ruby uses to find methods.

Summary

The method lookup chain shows where Ruby finds a method to run.

It helps understand method overriding and module inclusion order.

Use object.method(:name).owner and Class.ancestors to explore it.

Practice

1.

What does the method lookup chain in Ruby represent?

easy

A. The list of all variables in a class

B. The order Ruby searches to find a method to execute

C. The sequence of method calls inside a method

D. The order Ruby compiles the code

Solution

1. Step 1: Understand method lookup chain meaning

   The method lookup chain is the path Ruby follows to find a method when called on an object.

2. Step 2: Compare options with definition

   Only The order Ruby searches to find a method to execute correctly describes this as the order Ruby searches for a method to execute.

3. Final Answer:

   The order Ruby searches to find a method to execute -> Option B

4. Quick Check:

   Method lookup chain = method search order [OK]

Hint: Think 'where Ruby looks for methods' [OK]

Common Mistakes:

• Confusing method lookup with variable list
• Thinking it shows method call sequence
• Mixing lookup with code compilation order

2.

Which Ruby code correctly shows the method owner for to_s called on a string?

"hello".method(:to_s).owner

easy

A. String

B. Object

C. BasicObject

D. Kernel

Solution

1. Step 1: Recall where to_s is defined for String

   The to_s method for String objects is actually defined in the Kernel module, which is included in Object, and thus available to String.

2. Step 2: Check method owner for to_s on a String object

   Using "hello".method(:to_s).owner returns Kernel because String does not override to_s.

3. Final Answer:

   Kernel -> Option D

4. Quick Check:

   to_s owner for String = Kernel [OK]

Hint: String inherits to_s from Kernel [OK]

Common Mistakes:

• Assuming method is owned by String
• Confusing Object and BasicObject
• Forgetting Kernel defines to_s

3.

Given this Ruby code, what is the output?

module M1
  def greet; "Hello from M1"; end
end

class C
  include M1
  def greet; "Hello from C"; end
end

obj = C.new
puts obj.greet

medium

A. Hello from C

B. Hello from M1

C. NoMethodError

D. Hello from Object

Solution

1. Step 1: Understand method lookup order with include and override

   Class C includes module M1, but also defines its own greet method, which overrides the module's method.

2. Step 2: Determine which greet method is called

   Ruby looks first in C, finds greet there, so it calls "Hello from C".

3. Final Answer:

   Hello from C -> Option A

4. Quick Check:

   Method override in class wins over included module [OK]

Hint: Class methods override included module methods [OK]

Common Mistakes:

• Thinking included module method runs first
• Expecting NoMethodError due to confusion
• Assuming Object's method is called

4.

What is the error in this Ruby code snippet?

module M
  def hello; "Hi from M"; end
end

class A
  prepend M
  def hello; "Hi from A"; end
end

obj = A.new
puts obj.hello

medium

A. No error, output is "Hi from M"

B. No error, output is "Hi from A"

C. SyntaxError due to prepend usage

D. NoMethodError for hello

Solution

1. Step 1: Understand prepend effect on method lookup

   Prepending module M places it before class A in the lookup chain, so M's methods override A's.

2. Step 2: Determine which hello method runs

   Calling hello on obj calls M's method, outputting "Hi from M".

3. Final Answer:

   No error, output is "Hi from M" -> Option A

4. Quick Check:

   Prepend puts module before class in lookup [OK]

Hint: Prepend modules override class methods [OK]

Common Mistakes:

• Expecting class method to run instead
• Thinking prepend causes syntax error
• Confusing prepend with include

5.

Consider this Ruby code:

module M1
  def info; "M1"; end
end

module M2
  def info; "M2"; end
end

class Base
  def info; "Base"; end
end

class Derived < Base
  include M1
  prepend M2
end

obj = Derived.new
puts obj.info

What will be printed and why?

hard

A. M1, because include puts M1 before Base

B. Base, because class method overrides modules

C. M2, because prepend puts M2 before Derived in lookup

D. NoMethodError, info not found

Solution

1. Step 1: Analyze method lookup order with prepend and include

   Prepend M2 places it before Derived, then Derived, then included M1, then Base.

2. Step 2: Identify which info method is called

   Since M2 is first in lookup, its info method runs, printing "M2".

3. Final Answer:

   M2, because prepend puts M2 before Derived in lookup -> Option C

4. Quick Check:

   Prepend > class > include > superclass [OK]

Hint: Prepend modules come before class, include after [OK]

Common Mistakes:

• Thinking include module method runs first
• Assuming class method overrides prepend module
• Ignoring method lookup chain order