nyan.rb

module SerializeForSomeMiddleware
  module Nyan
    def serialize_for_middleware
      # Nyanは aの情報だけでよし
      {a: @a}
    end
  end
  module Wan
    def serialize_for_middleware
      # Wanは b も必要
      {a: @a, b: @b}
    end
  end
end

class Nyan
  def initialize(a, b)
    @a = a
    @b = b
  end
end

class Wan
  def initialize(a, b)
    @a = a
    @b = b
  end
end

def send_to_middleware(x)
  p x.serialize_for_middleware
end

nyan = Nyan.new(1,2)
wan = Wan.new(3,4)

nyan_for_middleware = nyan.extend(SerializeForSomeMiddleware::Nyan)
wan_for_middlewaer = wan.extend(SerializeForSomeMiddleware::Wan)

send_to_middleware(nyan_for_middleware) # => {a: 1}
send_to_middleware(wan_for_middlewaer)  # => {a: 3, b: 4}