set_current_comment.rb

class Post < ActiveRecord::Base
  has_many :comments
  
  attr_accessor :current_comment
end

class Comment < ActiveRecord::Base
  
  module Foo
    def and_set_current_comment
      self.current_comment = proxy_owner
      self
    end
  end
  
  belongs_to :post, :extend => Foo
end

# in code, lets say post 12 has 10 comments

@post = Post.find(12)
@comment = @post.comments.first

@post = @comment.post.and_set_current_comment

@post.current_comment = @comment