gistfile1.rb

class Blog
  attr_reader :entries
  attr_writer :post_maker

  def initialize
    @entries = []
  end

  def title
    "Underwater Basketweaving"
  end

  def subtitle
    "The best way to spend your time, all the time"
  end

  def new_post
    post_maker.call.tap do |p|
      p.blog = self
    end
  end

  private

  def post_maker
    @post_maker ||= Post.public_method(:new)
  end

end


blog = Blog.new

MockPost = Struct.new(:blog)

blog.post_maker = -> { MockPost.new }

p1 = blog.new_post
p2 = blog.new_post

[p1, p2].each { |e| p [e.class, e.object_id] }