One thing to keep in mind when extending Ruby classes

I was reading this post in Mauricio Fernandez's blog (rcov) and I got surprised about the behavior I'm summarizing in the following code:

class A
  def foo
    "A#foo"
  end
  
  def singleton_class
    class << self
      self
    end
  end
end

a = A.new
puts a.foo #=> A#foo
puts a.singleton_class.ancestors.inspect #=> [A, Object, Kernel]

module X
  def foo
    "X#foo"
  end
end

class A
  include X
end

a.extend X
puts a.foo #=> A#foo (and not X#foo!!!!!!!)
puts a.singleton_class.ancestors.inspect #=> [A, X, Object, Kernel] (and not [X, A, X, Object, Kernel])

So remember, you cannot extend or include a class with a module that is already present in the ancestors chain, it will be ignored!