A basic self-referential friendship model in Rails 4.0.1

friendship.rb

class Friendship < ActiveRecord::Base
  belongs_to :user
  belongs_to :friend, class_name: "User"
end

IRB.rb

2.0.0p247 :002 > u = User.create!(name:"test")
   (0.4ms)  BEGIN
  SQL (44.1ms)  INSERT INTO "users" ("created_at", "name", "updated_at") VALUES ($1, $2, $3) RETURNING "id"  [["created_at", Mon, 25 Nov 2013 19:35:06 UTC +00:00], ["name", "test"], ["updated_at", Mon, 25 Nov 2013 19:35:06 UTC +00:00]]
   (0.8ms)  COMMIT
 => #<User id: 1, name: "test", created_at: "2013-11-25 19:35:06", updated_at: "2013-11-25 19:35:06"> 
2.0.0p247 :003 > u.friends
  User Load (2.2ms)  SELECT "users".* FROM "users" INNER JOIN "friendships" ON "users"."id" = "friendships"."friend_id" WHERE "friendships"."user_id" = $1  [["user_id", 1]]
 => #<ActiveRecord::Associations::CollectionProxy []> 

migration.rb

class CreateFriendships < ActiveRecord::Migration
  def change
    create_table :friendships do |t|
      t.integer :user_id
      t.integer :friend_id

      t.timestamps
    end
  end
end

class CreateUsers < ActiveRecord::Migration
  def change
    create_table :users do |t|
      t.string :name

      t.timestamps
    end
  end
end

user.rb

class User < ActiveRecord::Base
  has_many :friendships
  has_many :friends, through: :friendships
end