Self-referential Associations in Rails

When I first heard about this concept, I needed clarification. I was reading Learn Rails by Example by Michael Hart

The concept was too complex to understand by then, so I researched. I found the Railscast episode Self-referential Associations, which shed some light on the concept.

I will show you the code for a simple self-referential association for friends and followers.

class User < ActiveRecord::Base
  has_many :reviews
  has_many :queue_items, -> {order(:position)}
  has_many :friendships, foreign_key: :user_id
  has_many :followers, class_name: "Friendship", foreign_key: :friend_id

  has_secure_password validations: false
  validates_presence_of :full_name, :email

  def included_in_queue?(video)
    queue_items.include?(video)
  end
end

Ryan Bates’s solution is probably more efficient than this one, but this one looks clearer to me. Ok, we have a User model and a Friendship model with user_id and friend_id columns.

Now is the time to link these two models. The line:

has_many :friendships, foreign_key: :user_id

Tells ActiveRecord to get all rows from the friendships table where friendships.user_id and the user.id match.

Here is the raw SQL query

SELECT "friendships".* FROM "friendships"  WHERE "friendships"."user_id" = ? [["user_id", 1]]

Thanks to the method has_many and some options, we can get what we want. The option foreign_key let us specify the foreign key used for the association.

The last line:

has_many :followers, class_name: "Friendship", foreign_key: :friend_id

Allos us to get the people following us.

Because there are no followers table, we have to specify the class_name in this case, Friendships, and the foreign_key option will help us determine the association.

Here is the raw SQL query

SELECT "friendships".* FROM "friendships"  WHERE "friendships"."friend_id" = ? [["friend_id", 1]]

There are probably better ways to do it, but this one helps me understand it better.