prepending-modules.md

Prepending Modules

When calling SomeClass.new.some_method, Ruby first looks for the method some_method in SomeClass, then Object, then Kernel and then BasicObject. That's the class's ancestor chain:

SomeClass = Class.new
SomeClass.ancestors # => [SomeClass, Object, Kernel, BasicObject]

When including a module into a class, that module comes right after the class itself:

SomeModule = Module.new

class SomeClass
  include SomeModule
end

SomeClass.ancestors # => [SomeClass, SomeModule, Object, Kernel, BasicObject]

Since Ruby 2.0.0 we can prepend a module to a class, which will place it to the ancestor chain's first position:

SomeModule = Module.new

class SomeClass
  prepend SomeModule
end

SomeClass.ancestors # => [SomeModule, SomeClass, Object, Kernel, BasicObject]

Note: The actual method lookup described here is simplified and does not mention singleton classes and other internals.