TIL #3 - Method look-up in Ruby

May 14 2020 • 2 min read

I stumbled across this TIL because I've been looking up advanced topics in a bid to learn new things and deepen my knowledge of Ruby & Rails.

# Super useful preamble

  • Everything in Ruby is an object.
  • All Ruby objects have an ancestor chain. BasicObject is always at the end of that chain.
  • All classes have a Class ancestor.
  • Class inherits from Module which means every class is sort of a module. "Sort of" because unlike actual modules you can't mixin classes. Module adds in instantiation, properties etc to classes.

Method lookup in Rails is possible because of a few fields all Ruby objects, classes, and modules have.


  • class: A pointer to the class object of the object. For example, for an Fish class, calling the class method returns Class.
  • iv_tbl (Instance Variable Table): a hashtable containing the instance variables of the object.

classes, modules

  • m_tbl (Method Table) - A hashtable containing all the class or module's instance methods.
  • super - A pointer to the class or module's superclass (direct ancestor). You can lookup the ancestors of an object by calling the ancestors method on that object.

# How method lookup works

  • Ruby follows the object's class pointer and checks its method table (m_tbl) for a match.
  • If there's no match, it follows the object's super pointer and checks its method table for a match. Ruby continues up the super ancestor chain, looking into each ancestor's method table for a match.
  • If there is still no match, Ruby invokes method_missing on the original object. FYI, method_missing is a method so it'll undergo this lookup process too until it is found and invoked.