Ruby Modules and method precedence explained

module_madness.rb

module A
  def foo
    "A"
  end
end

module Z
  include A
  def foo
    "Z"
  end
end

class B
  include A
  def foo
    'B'
  end
end

class C < B
  include A
end

class D < C
  prepend Z
end

puts "C.ancestors: #{C.ancestors}"
puts "B.ancestors: #{B.ancestors}"
puts "D.ancestors: #{D.ancestors}"

puts "C.foo=#{C.new.foo}"
puts "B.foo=#{B.new.foo}"
puts "D.foo=#{D.new.foo}"

# output

# C.ancestors: [C, B, A, Object, Kernel, BasicObject]
# B.ancestors: [B, A, Object, Kernel, BasicObject]
# D.ancestors: [Z, A, D, C, B, A, Object, Kernel, BasicObject]
# C.foo=B
# B.foo=B
# D.foo=Z