Understanding Rails ActiveRecord “single model” self joins

Question

I\'m having a hard time understanding how to implement a single model self-join in Rails. The Guide to ActiveRecord Associations section 2.10 briefly explains Self-Joins but do

Answer 1

I was missing the has_many foreign key to "parent_post_id". Once its set, the post.replies will reference other Post instances by parent_post_id.

The posts.rb model has the relationship defined:

class Post < ActiveRecord::Base
  has_many :replies, :class_name => "Post",
    :foreign_key => "parent_post_id"
  belongs_to :parent_post, :class_name => "Post",
    :foreign_key => "parent_post_id"
end

I can now create Posts, assign a parent_post_id to a different Post, then later get all Posts that are replies to any parent Post.