Because we used the scaffold generator for the posts and comments in our blog application, the Active Record design pattern is “pre-wired.” This means:

  • – A SQLite database able to store posts and comments was created when we ran the migrations.
  • – A connection to this database was established.
  • – The ORM for Post and Comment objects was set up — the “M” in MVC. However, there’s one thing missing — we need to ensure that any comments entered for a particular post are permanently linked to that post.

Associations in Rails

  • We did make the connection between posts and comments in the database — recall that a post_id can be stored with each comment.
  • To make our models in Rails fully functional we need to add associations — each post needs to know the list of comments associated with it, and each comment needs to know which post it belongs to.
  • There’s a many-to-one relationship between comments and posts — a post has many comments, and a comment belongs to a post:


  • The ActiveRecord module contains a set of class methods for tying objects together through foreign keys.
  • To enable these, you must declare associations within your models using the following:
    Relationship Model with no foreign key Model with foreign key
    one-to-one has_one belongs_to
    many-to-one has_many belongs_to
    many-to-many has_and_belongs_to_many