Modules And Lookup Path

This is based on The Well Grounded Rubyist book, you should definitely buy it

When you include a module in a class (or even in another module), Ruby inserts the module in the ancestors chain, right above the including class itself:

module M1
  def my_awesome_method
    "M1#my_awesome_method"
  end
end

class C
  include M1
end

class D < C; end

D.ancestors # => [D, C, M1, Object, Kernel, BasicObject]

Starting from Ruby 2.0 you also have a second way to insert a module in a class’s chain of ancestors: the prepend method. It works like include but it inserts the module below the including class, rather than above it:

module M2
  def my_awesome_method
    "M2#my_awesome_method"
  end
end

class C2
  prepend M2
end

class D2 < C2; end

D2.ancestors # => [D, M2, C2, Object, Kernel, BasicObject]