How to use Module.prepend to override class method in Ruby

In Ruby 2.0, you can use Module.prepend in place of alias_method_chain to override a method in another class:

class A
  def say_hello
    puts "Hello from A"
  end
end
module B
  def say_hello
    puts "Hello from B"
    super
  end
end
A.send :prepend, B
> A.new.say_hello
Hello from B
Hello from A

However, it wasn’t clear how to override class method:

module B
  def self.say_hello
    puts "Hello from B"
    super
  end
end
A.send :prepend, B
> A.say_hello
Hello from A

It turned out all you need to do is to call prepend on A’s singleton_class instead:

class A
  def self.say_hello
    puts "Hello from A"
  end
end
module B
  def say_hello
    puts "Hello from B"
    super
  end
end
A.singleton_class.send :prepend, B
> A.say_hello
Hello from B
Hello from A

(However, note that say_hello in the last example is now defined as instance method instead of module method.)