While monkey-patching a module from a Rails engine, we found that if you prepend a module B to another module A, the prepended module B won't be added to the ancestors of classes that have already included module A prior to the prepend. To illustrate:

module A
  def a
    puts 'a'
  end
end

class B
  include A
end

module C
  def a
    puts 'c'
  end
  A.prepend self
end

class D
  include A
end

B.new.a # prints: a
D.new.a # prints: c

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

Both classes B & D include A, but only D takes the new behavior as it's the one defined after we call prepend.

In contrast, we learned that class inheritance does not exhibit this behavior:

class A
  def a
    puts 'a'
  end
end

class B < A
end

module C
  def a
    puts 'c'
  end

  A.prepend self
end

class D < A
end

B.new.a # prints: c
D.new.a # prints: c

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

This time, both classes B & D have taken the new behavior, even if B was defined before we called prepend.

Is there a reason why prepend behaves differently when used with modules versus classes? I'm assuming this is by design, but it does present a gotcha when using prepend with modules.