有如下两个自定义的 model 类:
class Blog
  ...
  def add_entry(entry)
    @entries << entry
  end
  ...
end
# app/models/post.rb
class Post
  extend ActiveModel::Naming
  include ActiveModel::Conversion
  attr_accessor :blog, :title, :body, :pubdate
  def initialize(attrs = {})
    attrs.each do |k,v| send("#{k}=",v) end
  end
  def publish(clock = DateTime)
    self.pudate = clock.now
    blog.add_entry(self)     #   <=疑惑点?
  end
  def persisted?
    false
  end
end
不太明白 Post 中的 blog 是如何初始化为一个 Blog 的实例的,请高手指点或给个思路,谢谢~